Hi
If you do not have the original license file sent to you when you purchased it, contact the seller as they may have it on file. Otherwise contact Mach3 support as they should be able to sort you out.
Once you have the license file, place it into the Mach3 directory. This is usually C:\mach3
To get the Mach3 software download it from the Mach support site.
Cheers
Peter