Internet Sharing issue

[britishcolumbia]

I have a very "interesting" network setup, but it works. Let me bring you up to speed. My house is split up into two seprate foundations, with about a 15 foot gap (driveway) inbetween them. The internet connection is at location A, witch is a linksys router hooked up to a switch with about 10+ devices connected to it wired. It also broadcasts a Wi-Fi signal to my Airport Express in location B, witch sends the signal to yet another router with the home computers connected to it (Apple TV, iMac.) I recently got a MacBook air (no ethernet) and the wireless connection isnt very... reliable. I wanted to share the suprisingly speedy connection from the iMac (wired) to a wireless connection for the MacBook air. I have done this before, and know how to set it up. I turned on internet sharing through airport and set a password, the MacBook air recognized the network but when I try to connect to it the Macbook air just says "Connection Timeout." I cannot connect on my sisters MacBook pro or my iPhone either, so it is a problem with the iMac. Anyone else having this issue?