I'm a bit confused by this question because the license can be found in the following places
- The page where you
download OpenMPT- In the installed/extracted package, right next to OpenMPT.exe
- In the root of the source repository
- In OpenMPT's about dialog
I'm not blaming you for not finding it, but I'm curious where you did look to find the license and didn't end up finding it. Maybe there is some obvious place where it's missing and we should put it there.