From Pixelmon Wiki
Launcher crashes immediately before anything shows up
- Check C:\Windows\System32 for a file named "vorbis.acm". If the file is there, rename it to "vorbis.acm.bak", and start the Pixelmon launcher again.
- If you just upgraded your computer from Windows 7 to Windows 8, update your .NET framework to 4.6, or use this .NET Framework Repair tool. If you cannot install .NET 4.6 on Windows 7, you may have disabled automatic updates and your Windows is out-of-date.
- Check Windows' Event Viewer error logs (usually in Custom Views/Administrative Events) for more information about the crash.
- Report this information on the Pixelmon forums.