Hello CharlesC,
Thank you for your post.
Please try to uninstall all our products from your PC if there are several our products installed (including AVI Trimmer + MKV).
After uninstall please make sure that all files in the folders below were removed from your computer
"C:\Program Files\Solveig Multimedia"
and
"C:\Program Files\Common Files\Solveig Multimedia"
Then install AVI Trimmer + MKV again and try to edit your file.
Will this help you?