Back in the thread it's been said that mounts don't show up in File Manager unless mounted in /media. Normal /mnt mounts are hidden.

Also, you should make sure you can mount from command line, as it makes a difference if you can. Also back in this thread are other posts with similar issues, and walkthroughs on how to check if it actually works, including checking from Linux and Windows outside the N900.

Finally, what does the said network drive use as a protocol?