There are two kinds of usb to mini type cables. The old one is a bit fatter on the end that would connect to your N900, and that's probably what you bought. The newer one, which should be quite common, is flatter on the end that connects with your N900.

You probably need to reflash. You can follow Nokia's instructions for updating your software or there are instructions here at wiki.maemo.org on the same subject. There are also a zillion threads on the same topic.

Flashing is quite easy, but learning exactly how to do it may take some patience. Some people have problems doing it with 64-bit Windows 7, but it has worked ok for me.

PS: I just read your latest reply. Yes, Frys will have it.