Hi Andy,
Upon reading your more recent posts, i see your still having an issue..... and I realised you didn't mention doing any set up in your earlier ones...? 
You cannot simply plug and play the AE ....
Basically the AE should be able to join any wireless (non apple) network, but what you have to do is use Airport Utilities to do the set up. I believe this should have come with the AE and or you should be able to download it from Apple.
Launch the utlities and let it scan for AE devices, once its found your AE you select it and follow the steps to set it up 'to give it a name such as Living Room', 'to join an existing network" and to 'stream music' etc. during this process you will need also to tell the AE what your security system is (eg. a password if required) so that it can log onto the network... so you would need to confirm this prior to doing the setup.. however once this is done it will reboot the AE and you should now be able to now see the AE in your itunes, at the bottom right.
If I recall correctly Apple has some instructions and guides on its website to assist you.
Hope that this is the issue as this will be an easy fix.
If it is your security and it is based on MAC addresses you need to check with your IP provider and they should be able to give you a web address to go in and adjust / add MAC addresses on your router for your network and or change it to a password WAP/WEP etc system.
Trust some of that makes sense for you, I am a bit of a novice myself, but went through a lot of these issues to get my network functioning properly... so not sure if I am explaining things correctly.
Laurence.