Initially the download copy of Teacher's Personal Markbook for OS X acts as a 30 day trial version. But after the trial period you can open markbooks and print from them, but you cannot save any changes that are made. If your installed copy of TPM is a trial version and will not save, please check the number of days shown in the When does this trial expire command in the Help menu. Once the number of days is zero then you must purchase a licence to register your copy to allow continued use.
If you have copied TPMX to a different computer you may need to re-enter the License name and key in the Register command in the Help menu to re-register the program.