Aldevil:

1. The N900 is not on the officially supported MMS device list, so when it tells your carrier's network that "hi, I'm an N900, wanna let me in?" the network checks that no, you have not been sent the MMS config but as the N900 is not on the officially supported devices' list, no message ever gets sent.

2. There's a handy app called fAPN in the repos for adding the necessary APN.

Edit: duh, someone's always faster than me