It sounds like you are using the old Cydia installer that works on the 1.1.* firmware versions. That version does NOT have the possibility to add other sources.
If you however update to firmware 2.0 and use the new Cydia installer you can add sources. Note however that the new Cydia does NOT support the old 1.1.* firmware
So i recommend that you use pwnage to update your phone to 2.0 or 2.0.1.
You can read about this in other posts here on modmyi or at
iClarified - Tutorials - iPhone