My main network in a 802.n on a Airport Extreme Base Station. I want to add an Airport Express to stream music. What is the difference between "Join Existing Network" and "Extend Existing Network?"
iMac Core 2 Duo [2.33ghz]/15" MacBook Pro [2.16 ghz],
Mac OS X (10.6.2)
In order to do what you want you must configure your AEBS to allow its network to be extended. Open Airport Utility, select the AEBS, and click on the Manually button. Click on the Wireless tab then check the box in the panel labeled "Allow this network to be extended."
When you configure the Airport Express you will set it up to "Join Existing Network" if you are streaming music or "Extend Existing Network" if you are using it as an access point to extend the range of the AEBS.
It's a question of task. If you are planning to stream music then you want to join the existing network. You can opt to use the extend network option, try streaming music. If it seems to be working as you want then leave it. If not you can reconfigure the AEX to join the network instead.