Camtasia for Mac 1.2.2 changes where the registration key is stored; it is now placed into /Users/Shared/TechSmith/ which is accessible to all users on the computer. You should no longer get this error when using v1.2.2 or any later version.
If this issue persists after updating to our current releases, there may be a permissions problem in your Shared folder. Delete the contents of the /Users/Shared/TechSmith/<application name> folder and re-enter your software key. The /Users/Shared folder should be set to read/write for all user accounts.