I installed Hitfilm Express, have an account, and have edited several projects in "demo mode" yet when I open the activation app the app crashes.
OS X 10.9.5
You may need a support ticket.
Patience there. I think the office is closed till Jan 3.
What year is the Mac Mini? That will help our tech inclined users determine your exact hardware. Someone else might have an idea. I got nothing.
For anyone else coming across this thread, there is a known issue with activation and OS X 10.9.5. There is a workaround, which has been successfully tried with one user. I won't post the details here though as we need to confirm the problem and work through it with users directly - so as Triem says please contact the Support team.
However, bear in mind that OS X 10.9 is now outside our official supported range of operating systems, so there may be further problems which we are unable to resolve.