Force iOS simulator to use WiFi network
The iOS Simulator does not get its own internet connection, it uses the computer's. Therefore, if you want it to use the Wi-Fi network instead of the LAN, just make sure you're connected through Wi-Fi.

In System Preferences / Network you can rearrange the order in which the computer gains Internet connection. Just place Wi-Fi on top of the list. On Lion it's done by clicking the gear thingy, and select Set Service Order from the pop-up list.