Recent versions of OS X El Capitan may refuse to open MultitrackStudio "because it is from an unidentified developer". Apple changed the code signing rules, and this somehow refuses to work with MultitrackStudio. For now you can install MultitrackStudio like this:
Control+Click on the MultitrackStudio app in Finder.
Click "Open.".
A new popup will appear, asking if you're sure you want to run the app. Click "Open" again.
That trick, however, does not seem to work with latest 8.3 update.
After unzipping the MultitrackStudio-Standard-OSX.zip file and copied it to
Applications folder, the attempt to launch it shows a dialog with the message
<The application “MultitrackStudio” can’t be opened.>
My OS is 10.11.5 (15F34)
It works for me. I suspect you didn't actually perform the trick: did you click while keeping the Ctrl key down (or just rightclick), and then choose 'Open' from the menu? Simply clicking the app won't work.