custom clock format variable in message box

You can use following variable assign :

session.av_time_human_readable = clock format [mcget {session.check_software.last.av.item_1.db_time}]

or

session.av_time_human_readable = return [clock format [mcget {session.check_software.last.av.item_1.db_time}]]

but not with brackets starting line.

then in the message box, use following text:

Last AV DB time is %{session.av_time_human_readable}

to answer to the other question, APM can't read irule variables, only session variables. so after executing the irule code, use ACCESS::session data set command to store irule variable to session variable.