on my iMac which I just got, click the wireless icon on the top right.
It should show a list of wireless networks it detected. If you configured
a name for your wireless AP it should be easy to spot, otherwise it would
some generic brand/model name. When you select your wireless network
it should prompt you for the passwd.
Initially my security was WEP-Shared which my iMac couldnt attach to.
But changing to WEP-Open made it work.