I was having similar issues and contacted support. Their response was quick and works great! Here you go:
Hi, and we're sorry about the trouble you've run into with your system. Since the release of Apple’s Yosemite OS, we’ve seen a relatively small number of reports that Line 6 Monkey does not open on some systems running Yosemite. This is a system configuration issue whose result affects not only Line 6 software, but possibly other software on a given computer.
Please move or delete any .plugin file from the Library/Audio/Plug-Ins/HAL folder, then restart your computer and open Monkey. Please let us know the results. Best regards, Jelena Line 6 Support
