What Developer asks is a good question; however, in case you don't know what a creator code is, here's a really easy way to see if that's the problem (and fix it if so).
1. Download XRay from VersionTracker. This is a great app for examining and changing file info.
2. Drag one of your Quicken files on XRay.
3. Click on the "Show" pop-up menu and select "Type, Creator & Extension."
4. Look and see what application is selected as the Creator. If it is something other than Quicken, click on the pop-up menu and change it to Quicken.
5. If you had to change the creator, chances are the type was wrong too. Click on the pop-up menu for Type and you should get a list of the types that Quicken supports. Usually, there's one of them that pretty much gives itself away as the one you want from its description. However, if there isn't, you can just try a bunch of them until you find the one that allows the app to be opened in Quicken.
6. After you change this info, Save in XRay, and then try to double-click the file in the Finder. If it works, then you have succeeded! Now you just need to make the same changes to the rest of your files. Fortunately, XRay makes this easy. Just put all the files you want to change into the same folder, and drag that folder onto XRay. Go to Type, Creator & Extension again, click "Change type and creator of enclosed", make the changes you want, and presto! You should be good to go. Make sure there aren't any files other than the Quicken files you want to change in the folder!
7. Send the author of XRay the registration fee, because his program rocks.
Hope that helps,
Charles