It sounds as though you have probably uninstalled incoreectly in the first place... not your fault as I don't think we are clear enough about that entries available in the windows control panel.
I'd do a manual clear out and then reinstall. Manually delete these folders if they are still there (back up your Termbases first in case you make a mistake):
c:\Program Files (x86)\SDL\SDL MultiTerm\MultiTerm15\
I think that's about it for a clean install. Then reinstall.
If you still have problems after this I'd recommend you log a ticket with support so an engineer can look at this for you. This is clearly an installation problem and for licensing or installation issues that prevent you from working we have a route for all customers to get help. This can be achieved really quickly using this link:
Just fill in the form and submit it. Make sure your email address is entered correctly or nobody will be able to get back in touch with you.