A little success!
I rebooted the phone and tried a few more times, and now the Windows mount is showing up in the file manager.

Two potential sources of problems were caused by the N900 automatically capitalizing the first letter of the username and password which appear to be case sensitive and therefore would not work. After forcing each of those to all lower case, I was able to mount successfully.

Also, after adding an entry to the Hosts file with the IP address and the name of the Windows system I was trying to connect to, the name can be used (instead of the IP address) in the Server Address line.

I mounted in /mnt based on the commands suggested in this threat. I haven't tried media yet, although if that is what WM uses, this might account for why it shows up now that I fixed the password issue.