If your MT router is PC (x86) based then either Telnet into the system or use a keyboard and monitor connected directly to the PC.
If your router is RouterBoard based, then your only option is to Telnet in. I think it also has a serial console port, but as all my MT's are currently PC based, you would need to refer to the documentation on how to use a serial console port, it's much easier to use Telnet.
You can use the Telnet option in Winbox to connect to the CLI.
Once you've logged in at the CLI (Command Line Interface) using your admin password then type in:-
/tool user-manager customer add login="admin" password="adminpassword" permissions=owner
but of course substitute whatever username and password you want to use. Only change the parameters in quotes, everything else must be exactly as shown above.
This creates an admin user that is specific to User Manager and is completely independent of the admin userid for Winbox and Webbox.
If this doesn't sort you out, please elaborate on your problems, it's hard to guess what's wrong as I can't see your monitor from here!!