Use the Issues feature

Add a new issue

You need a Google account for this but from a developers POV it's much better than emails (also, other people can see the reports and can then give additional info.) Additionally, it gives a nice template with helpful questions regarding to what happened, which will make the whole fixing process much faster.

If possible, include the song/instrument files that crashed and also the configuration file

The configuration file can be found as .klystrack (note the dot) in your user directory. In Windows this is usually C:\Users\kometbomb\.klystrack and in *nix it's the home directory. The files can be attached in the issue (described above.)

Even if it didn't crash, you can still report a defect

Bad software stays bad if the users don't suggest new features.