Thanks for trying to evaluate Snip-It Pro. I am not exactly sure what could be the cause of your issue, but suspect that your download was corrupted somehow, (only because I knew of one other person that had a similar error)
What is the file size of the installer?
It should be approximately 1.42 Mb.
You can try downloading it again, or if it continues to be an issue I can email you the installer (zipped up).
Just send an email to email@example.com
Sorry for any inconvenience this caused you, let me know if I can be of further assistance.
-David San Filippo