As for the topic at hand: it seems the best way to go is to install Pidgin (not the j2me thingie, ugh), and just use Pidgin to connect.

http://forum.mxit.com/viewtopic.php?...8c65fc1af662d1 has even a full PDF file with pictures for adding MXIT connection. It is aimed for desktop Linux, but with a few minor differences it should all work the same on N900's Pidgin.