It does show up in the default file manager even if it's formatted NTFS(probably other fs too), although it gives an error msg that the file system is not supported.

I would say check if you inserted the card correctly and reboot N900 then see if it works fine. (removing cover will unmount the memory card so try to keep cover on while testing)

Sometimes upon leaving mass storage mode N900 does not(can't) mount memory card, although it's very rare, but happens. In this case a reboot will sove the issue.

PS: Welcome to the light side