It’s possible that the presets were compatible with CS3 but CS5.1 changed something that makes them incompatible. The “or higher” is more telling you the minimum version not a guarantee that Adobe might change something in the future.
Are these presets free and open so you can share here via dropbox or similar service or are they private? It’s hard to give a more specific response without seeing the presets involved.
What are you doing to “upload” (I would use the word "install" or "use") the presets that isn’t working? Have you placed them where they're supposed to be, and CS5.1 is throwing an error, or are you not sure where to put them and CS5.1 is not finding them or what is the situation?
The presets came with a class workshop, so not for public. They came in a zip file that when you click on it atn file (zip file). They loaded into the action section but say:
Step 1 Run this action
below that box it says
Click here to start
(When I do nothing happens)
I see. Make sure that you've actually unzipped the ATN file(s) out to their own folder, instead of just clicking on something still in the ZIP file, as there are other files that may be necessary. Is it possible there is a readme file that tells you some activity to perform before the action works?
Maybe something would be more obvious if you provided a screenshot of what the action looks like with it's steps expanded