The compatibility issue has more to do with the Belkin router than the airport express.
In order to join (or extend) a wireless network, the original network has to be set up to allow WDS. Some wireless routers allow this by default, some allow it to be set if needed (such as all apple wireless kit), while others do not allow WDS at all.
If your router is one of these there's nothing you can do, other than turn the wireless off and connect another express or and extreme to it via ethernet and use that to set up a wireless network that allows WDS.