The Design menu is only available if the Steel Designer codes have been selected during Multiframe installation and the Steel Designer licenses have been activated from the Edit->Preferences->Licensing dialog.

Licensing is very much a first come, first served situation. Assuming both users select the steel design codes at installation, basically whoever starts Multiframe first and requests the steel designer licenses will get them, the second user that starts Multiframe will get a message to the effect that there are not enough licenses available for those steel codes and those steel codes are automatically turned off. To turn them back on the user needs to reselect them in the Edit->Preferences->Licensing dialog and restart Multiframe before it will attempt to obtain those licenses again.