In order for Papers to function properly it has to have read/write access to its Papers library. Often when you copy the library from one mac to the other the Papers library folder can end up with the wrong permission settings.
In order to verify whether your library folder has the right settings do the following:
- in the Finder locate your Papers 3 library folder (library.Papers3).
Note that if you use OS 10.7 Lion, to access your Library folder, you will need in the Finder to use the Go menu while pressing the option key, which will add a 'Library' item to the list.
select Get info from the File menu and scroll down to the Sharing and Permissions section
the table should show your account ("Me") as having Read & Write permissions
If this is not the case continue with the following steps:
click the lock to make changes, enter your password if necessary to authenticate
click the plus button below the table and add yourself from the list of Users & Groups
next, click the small cog wheel button and select Apply to enclosed items...
Your library should now have the right settings