I am having exactly the same problem.
Can you please show how you used the "alias" to solve the problem?
I will really appreciate it, since I used to use this script all the time in CS4 and all of a sudden it doesn't work.
It seems to be a problem with one line of code.
To fix edit the ImageCatalog.applescript using the AppleScript Editor. Once you have the script open and are able to edit, do a search for: as string
once you found the text "as string" change it to: as alias
save the script and it should work now.