I just spent 2 days trying to figure this out and I got it.

On th Mirror reference the Primary server.

ALTER DATABASE MYDB_04

SET PATNER = 'TCP://PRINCPLESERVERNAME.DOMAIN.LOCAL:PORT#'

On the Primary reference the Mirror server.

ALTER DATABASE MYDB_04

SET PATNER = 'TCP://MIRRORSERVERNAME.DOMAIN.LOCAL:PORT#'

This is not stated correctly in the documentation the I have read.

K9