No version of InDesign can open PM4 or PM5 files. Only PM6 and later can be
opened by CS6 and earlier.
All that said, if you can't activate it or have a serial number issue you'll
need to contact Adobe directly.
It sounds like you've been scammed. It appears you bought software on the internet somewhere, and it doesn't have a valid serial number.
Furthermore, CS3 (or any version through CS6) can ONLY open Pagemaker 6.0 and later files.
Steve, you may be right, but the OP doesn't give a clear picture of what he/she did or has. The phrase "in the original box" could mean an unopened box (in which case they have not been scammed), or it could mean an opened box that has been used and registered. If that used and registered copy has been used as an upgrade path for the seller, it can not be resold, but if it has not been used as an upgrade path, I believe it can be resold if the seller has removed it from their computer and is no longer using it. The fact that they didn't deactivate it is a problem for the buyer, but could be corrected if the buyer can contact the seller and have them contact Adobe to deactivate it, but that's probably not going to happen.
And you are correct that they would need not only PM6 to convert the PM 4 files that could be converted to ID files, but also a computer and OS that will run PM6 (which is 90s technology).