Connect to Comcast cable internet on my Mac Desktop

Use a CAT-5 ethernet cable (probably came with your cable modem) to connect the modem to the Mac (there's a spot on the back where it fits). Go to System Preferences, and click on the Network icon. Highlight the item "Built-in Ethernet" in the left-side panel (if it's not there, click on the '+' below, and select 'Add a new service', and create an Ethernet one). Look on the right-hand side where it says 'Configure:' and change the drop-down to 'Using DHCP' if it doesn't already say that. Click the 'Apply' button if it's not greyed out. If the dot next to 'Built-in Ethernet' is not green, click on the little gear icon below and select 'Make service active' from the list.

I would also suggest that you consider going to System Preferences and clicking on the 'Security' icon. Select the 'Firewall' tab, and then check 'Allow only essential services'.

Wireless is quite similar. You plug your wireless router into the cable modem (it should come witha short cable for that purpose, and it should have a jack marked 'to cable modem' or 'gateway' where it goes). Connect another cable between a computer and the wireless router and configure it per the router's manual. Generally speaking, that involves logging in with a default username and password, setting a new password, and selecting the security options you want to use.

You can disconnect the computer.

On the Mac, go to the network control panel and click on 'Airport' (the internal wireless card). Click the 'Turn AirPort On' button. Select your router where it says 'Network Name' -- it's what your router's manual calls the 'ESSID'. Your router probably allows you to check a box to hide the Network, in which case you need to select 'Join other network..." from the Network Name drop-down list. The Mac should automatically detect whether or not you've secured your router and which option you used, it will then prompt you for the wireless password that you setup on the router (if you set one up). Click 'OK' and you're done.

Do you have an Ethernet wired router or is the only router you have the Airport Express? If the latter is the case, then you want to do the following:

Disconnect any cables from your computer's Ethernet port. Open Network preferences on your computer. Select the Airport port and click on the Advanced button. Set the Configure IPv4 drop down menu to DHCP and set the Configure IPv6 drop down menu to Off. Click on the Apply button.

Turn off the computer, the cable modem, and the Airport Express. Wait at least one minute. Turn on the cable modem and wait until the status lights indicate a connection with the ISP. Connect an Ethernet cable from the WAN port on the cable modem to the Ethernet port on the Airport Express. Power up the Airport Export express. Turn on your computer, and turn on Airport from the menu bar icon. Open Airport Utility in the Utilities folder and configure the Airport Express to create a network. The status light on the Airport Express will eventually turn green when the connection is established to the cable modem.

I've been following this thread and noticed that no one had mentioned a modem...

Did you get a cable modem from Comcast or did you buy one elsewhere? Before you can use a router, you need a modem - that's the piece of equipment that will "talk" to Comcast and get you on the internet. Once you connect the modem (with the CAT5 cables usually supplied with the modem) from the wall plug to modem and then from modem to Mac), call Comcast and they will "read" and register the modem on their end and it should work. Instructions are also included in the box the modem came in.

If you want to use a router, the CAT5 has to go from wall to modem -> from modem to router and -> from router to Mac. In that case, you still need to contact Comcast to register the router as well and you will have to set up the router (usually online) before it'll recognize the Mac.

OK, so your Mac is recognized with the cable and you can get online, that's good.

You said that your Macbook connects "through airport to my comcast cable", but earlier you mentioned that you didn't have an Airport Express (router), so I'm a bit confused. The built-in airport card is not a router, it just enables the Mac to pick up a wireless signal from a router. Do you have a router?

If you do have a router (or intend to buy one), included instructions will tell you how to set it up. If it has a Mac compatible setup CD included, it's easy; some do not and you need to go online (again, follow their instructions) and set it up manually including entering any or all of your computer's MAC addresses - without these steps, the router will not recognize the computer.

As for the network adapter, a signal from any wireless b device will not have very far reaching coverage, so that could be why Comcast blamed it on the layout of your house. Most routers or adapters today are g and/or n enabled which is newer technology and gives you much better coverage/signal. I do not own a Linksys product, but browsed their support pages; unfortunately, they no longer list any wireless b adapters, but the instructions for their g and n enabled adapters would be a good source of information on how to configure your adapter and join a wireless network, so you might want to check their support page. In any case, you would need to first establish a wireless network in your home and for that you need a router.