You're not doing anything wrong. The issue you’re seeing with CheckBook Pro is due to the version you have. It's something that has been brought to our attention and will be fixed with the next update.
Basically, just the CheckBook Pro update submission that got loaded for the Mac App Store still contained a piece of iffy code related to Summaries. That had gotten fixed but there was just a mistake made on our part such that previously compiled application files got sent to Apple. We resubmitted the update files with the fix you need to Apple and so keep an eye out for the next version to show there. As soon as the next update is installed, everything will work just as expected to.
If you need to generate some Summaries now, you could download the version from our website—that version doesn’t have the issue you’re seeing and will accept your App Store licensing. Here’s the link for where to get that:
http://www.splasm.com/checkbookpro/update.htmlCheckBook Pro 2.5.4 at our site could even be installed alongside your App Store copy. In doing this, you could just use that version until the App Store update shows up and then remove it.