Licensing can play a roll. Make sure in the MAS that 1 - you have an account setup for yourself. I guess you've confirmed that by using the web front end.
Depending on what version you are using, you may want to uninstall and reinstall so you check the URL you are using for pointing UCA to the MAS. In my case it's simply:
mitel.domain.org
no http or / or anything like that. simple. If you give yourself a smartphone license for mobile uca, there's a test connection button that might be helpful. You should test the mobile app.