There is also a .CNT file that the User Manager EXE file needs to reference. Probably your shortcut Run In path is not pointing to the right spot for this other file to be referenced.
You can copy/paste the User Manager files from a NT Server cd and put them on your local workstation, and run it from there.
Take a NT Server cd and browse to:
\CLIENTS\SRVTOOLS\WINNT\I386
Copy everything in there (2.28MB of files) and put them on a local directory on your workstation.
Then, make a shortcut from this copy of USRMGR.EXE and put it on your desktop or wherever you want it to be.
Double-click it.
It will take a moment for it to synchronize with the PDC/BDCs on your network. If it gets angry, saying a domain can't be found, click Ok onthe error box and then it will ask you for the domain to administer. Select your domain and click OK.
That should do it. The next time you open it, it will run just fine.
The rest of the files from this directory are also useful to run locally: DHCP Manager, Policy Editor, RAS Admin, Server Manager, WINS Manager, Remote Boot Manager and the Macintosh services manager.
You don't need to run these application only from a domain controller.
hope this helps.