We have seen that Scarlett Mix Control can crash when using OS X 10.9 Mavericks. Our software team are currently working to investigate and fix the issue.
We have found a workaround to the problem that will prevent the software from crashing and will allow users to continue using Scarlett Mix Control. To do this first please ensure that mix control and all audio and midi applications closed, including web browsers.
Then go to Applications > Utilities > Activity Monitor and in there please find the MIDIServer process. If you cannot find the MIDIServer process please try and open Scarlett Mix Control and you should find that it opens. If the MIDIServer process is listed please click on this and then once highlighted please click on the black X symbol in the top left of the window. This will kill the MIDIService process.
Once the process is stopped Scarlett Mix Control should open without an issue. If you need any further assistance please contact us here.