Does this happen with every image you try?
When you select the image using 'Theme-It' and click the fireworks icon, do you get the 'Find Source' dialogue box when Fireworks opens? Which option are you selecting, ('Use a PNG', or 'Use This File')?
The 'Use This File' option should work - Is this where you are getting the error message?
Try using the 'Use a PNG' option, and browse to find the image file on your PC. That should get you by until someone posts with the proper fix.
NOTE: If you don't get the 'Find Source' dialogue box in Fireworks, enable it in FW Menu - Edit > Preferences > Launch And Edit > and choose 'Ask When Launching' from the select lists.