How to use JobMinder AssignmentData?
Usually your editor sets up the JobMinder assignments for you. Are you an editor?
How to use User/Client variables?
You fill in the fields in the User/Client Variables dialog. Then you use the individual variables like {username} just like any other variables you would normally use. (Like in the IPTC Stationery Pad, renaming, folder naming, etc.)
Are IPTC Core fields planed?
If you mean things like the "Location" field, yes. In 4.5.
Manual?
Coming soon.
-Kirk