I don't think it is on your computer. Do you have a router? If so, I think the default IP address is 192.168.1.1 so you input that into your web browser and the router page should come up. if it asks for a password then you'll have to put the password in (default may be "admin"). Then browse for wireless security settings and check what the WEP password is set to.
The WEP key (although better than nothing, a WEP key can be decoded in 15 minutes) is a phrase you type into your computer, which has to match that on the router. The only network-related information on your computer is the Airport ID, which can be used at the router to allow access via the Airport ID (also called a MAC address).
WPA is a more recent encryption standard that is much more secure than WEP, which Apple supports. I'd recommend using it if you can set your wireless router to support it.
If you are trying to connect to another wireless network that isn't yours, then they have a wep password for a reason, lol. A florida man was arrested for using someone else's wifi connection, so beware.