System.AggregateException: One or more errors occured ---> System.SystemException: RegisterServicePort(com.xamarin.MTHosting.2a0b1, com.apple.PowerManagement.control): Kernel returned: -308 (-308): (ipc/mig) server died
Errors with "RegisterServicePort" and similar error messages like above are commonly an issue with spyware/malware on the computer. Please consider the comment on this bug report for more information, along with the link to the Apple forum discussion on how to remove a possible infection.
To assist in diagnosing the issue, open up the MacOS application 'Console' and delete every file inside the 'User diagnostic reports' section http://screencast.com/t/y9i3NKcuMy. Then start Xamarin Studio and try to use the designer. If any new log files appear in this section after the designer has failed to initialize, please save these for us to analyze.
Please note the most important thing to check for is this file:
No matter the above results, if that file exists, the aforementioned spyware/malware issue is present on your computer.
The following link has the steps to remove this spyware/malware: http://www.thesafemac.com/arg-genieo/