In addtion to the steps to follow in HOWTO54944 Licensing an unmanaged client, after the .slf file is placed in the client's inbox as described in the HOWTO article, check the client's system log.
In the client's UI - VIEW LOGS>Client Management Log>System Log. An entry appears that states "The client has successfully applied a license file (nnnnnnnn) from the inbox.", where nnnnnnnn is the name of the license file.
Some time after the license file is applied, another entry appears - "[Client authentication token request} Information submitted to Symantec] Size bytes (nnn)" where nnn is the length of the CAT request.
If a valid license is not present, the CAT request will fail.