Where is a missed approach point for circling while flying ILS?
Circling approaches are non-precision, regardless of how you arrive at the MDA.  When circling you'll level off at the circling MDA and use the missed approach point depicted for the approach.  

If you are looking at a Jeppesen approach plate there should be a level-off and 'M' depicted in the profile view.  If you are looking at government plates the level-off and MAP are not depicted in the profile view but are still specified elsewhere.  In any case, you'll fly the localizer to the MAP and then go missed.  You'll descend either via the GS or step down altitudes to the circling  MDA and stay there until either the MAP or you are able to further descend.

RVR probably won't be listed for circling minimums (it isn't published for the chart as a whole) as they will generally be at least 1 mile or more.  Whether the visibility requirement is specified in feet (RVR) or miles, this is the visibility you need to descend out of the circling MDA.  You have the further restriction that you may not descend out of MDA until you are in a position to land.  The same familiar rules in 91.175 still apply. 

In summary, fly the approach like you normally would but level off at the non-precision circling MDA and go missed at the non-precision MAP if you have no visual contact with the runway.  You do not go missed exactly upon reaching the MDA, just as you wouldn't if you were flying a VOR or LOC approach.  You go missed at the missed approach point.