How did you install CSSU-thumb? Did you use the install file?
Using microb on your n900 click here and follow the instructions on http://wiki.maemo.org/Community_SSU, section Installation, from step 5 on (Hildon Application Manager (HAM) will launch...). cssu-thumb is based on cssu-testing flavor.\
Did you add the repositories manually? Did you try to re-import repo settings into fapman application?