There's no need to import PodPress data, what you can do is use the option in the Basic Settings of PowerPress, second option titled PodPress Episodes, and select "include in posts and feeds". You can convert PodPress data to the WordPress enclosure format either after you have WordPress 2.8.4 installed or if you're happy with the way things are, you never have to import the PodPress data.
I'm looking into the error you reported with the function, it may be that a file was not uploaded in your WordPress installation. Navigate to all the other pages in the admin area such as the Posts, Comments, Pages, etc... pages and make sure you do not receive the same error message. I'm doing testing on my older copy of WordPress to see if I can duplicate the problem as well.