Connecting Mac to Mac

I have an iMac and a MBP.

Are they both running OS X Yosemite 10.10.1? Or a newer or older version of OS X?

I have a Sky Broadband modem, and I also have an Airport Extreme connected, so I have 2 networks I can connect to.

Is you goal to have two wireless networks or would like assistance in just making them provide a single network? To be able to share resources between network devices they must be on the same network.

Finally what is your ultimate networking goal for your Macs?

At this point I am assuming that your Sky Broadband modem is actually a combination wireless router and modem (or wireless gateway) device. Regardless, what is its actual brand and model number? There are a number of options to creating a "single" wireless network and the modem info would help. Finally, which exact model of the AirPort Extreme do you have?

The following Apple Support article should provide you with some ideas on how to go forward, but we will be happy to help if you need further assistance.

How you configure your 3rd generation 802.11n AirPort Extreme base station will depend on whether the Fibre broadband-supplied device is either a simple modem or a combination modem/router (or gateway). Either way the base station would need to be connected to this new device via an Ethernet cable.

Also if the Fibre-provided device provided wireless then you can also decide if you still need to use the Extreme at all.

Unfortunately, extend a wireless network is the wrong setting.

Please let the setup utility suggest the setup for you. It will be correct.

Start over again by holding in the reset button on the back of the AirPort Extreme for 7-8 seconds, then release and allow a full minute for the AirPort Extreme to restart to a slow, blinking amber light status.

Insure that the Ethernet cable connects from a LAN <--> port on the Sky modem/router to the WAN "O" port on the AirPort Extreme.

Click the WiFi icon at the top of the Mac's screen and look for a setting of New AirPort Base Station. Just below that, click directly on AirPort Extreme.

That will get the setup wizard running. It will take a minute to analyze things and then present a window that looks similar to this, except that you will see a picture of the AirPort Extreme:

Network Name........Enter the same name that the Sky device is using......IF.....the AirPort Extreme will be located a few rooms away from Sky device. Enter a different name....something simple of your choice......IF.....the AirPort Extreme will be located in close proximity to the Sky device.

Base Station Name......Enter a name that you want to call the AirPort Extreme. Mine is AEBS, for example

Password........If you entered the same wireless network name as the Sky network above, then enter the same password as the Sky network. Otherwise, choose a different password that you want to use for the network that you are using

Verify.....Verify the password again

Click Next

The wizard will setup everything for you. Wait a minute for a message of Setup Complete, then click Done. And, you are done.

Extend a wireless network would be the correct setting if the AirPort Extreme were connecting using only wireless to another Apple router.

But, you are connecting using Ethernet, so create is the correct setting.