When we first purchased the FMG had a Fortinet Engineer onsite to help us with getting the Authenticator online so I stole a couple hours for the FMG.
However the best advice he gave was a either start using it on a lab unit, or in our case, create a couple of extra VDOM' s on our DR cluster and play with those.
But if you' re comfortable the Fortigate interface it isn' t too hard to pickup the Managers interface, just don' t overreach, ie; don' t go turning on all features at once.
And for specific questions these forums are great.
Regards,
Matthew