Reset your preferences by deleting the following:
- Quit iShowU HD first (if it's running)
- Delete /Users/<you>/Library/Preferences/com.shinywhitebox.iShowUHD.plist
- Delete /Users/<you>/Library/Application Support/iShowU HD/iShowUHD.xml
- Restart iShowU HD.
If it is still giving you the error message please submit a support request
p.s: If you're using Lion and can't see the "Library" folder, this post should help: