*** Improved by Wrobinson on 9/8/08 ***
There is not a predetermined pre-shared key. Instead, you create one that is compliant with the encryption standards of your wireless networking equipment. It is not uncommon for wireless access points to allow you to generate pre-shared keys through the user interface. Once pre-shared key encryption is enabled on your wireless networking devices, each device needs to be configured with they key in order to communicate on the wireless network.
Like Schmidtw mentions below, you should look for the pre-shared key in the same area as wireless security options.
*** End update ***
The preshared key should be on the same page of the web-interface as the router’s security page. You will have to access this on a browser by typing in the IP address.
Hope this helps!