I have been a Herald level judge for over a year, looking to advance to the next level, Master. The master level test is not available online. How do you proceed to the next level?
The levels of RPGA gamesmaster do not really exist anymore, as far as I can tell.
You do not even have to have passed the Herald level test to organise RPGA events, now - any WPN organiser can do it.
To be honest, the two levels which existed when I joined the RPGA in around 2004 didn't really do much: the Master test was all about organisation of larger events so didn't reflect judging ability at all, so you're not really missing anything!
Pedr has the right of it.
The "advanced levels" of RPGA GMs are a thing of a bygone era - and were mostly holdovers from the days when GMs were organised in a completely different near-tournament-like structure.
I think, officially, we're all WPN Event Organizers now - as the RPGA "brand" is evolving to find its place in the WPN structure. The Herald test, for example, is no longer required (but still feels "right" IMHO - even if one doesn't need it). 
The Herald test always felt like a pointless waste of time, rules knowledge does not a good DM make (especially in 4E).