fMMS was a nightmare for me as well. I flashed few times to get to know how to get it to work.

THIS IS A MUST-DO STEP
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Xterm:
sudo gainroot
gconftool-2 -st string /apps/fmms/useragent "New UA String"
(replace "New UA String" with any Nokia UA)
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If u do the step correctly, any APN use, be it openmms or others, it seems like each password work for any user name! Its hard to mistake when u do the step above right in the 1st place.

Good Luck!