I make it five (or 7/9/11 etc if the new point is on P2/3/4 etc of the flight plan).
ATC: Direct XYZ
Press LSK next to XYZ
Press LSK 1L
If there is more than one XYZ then the FMC will give you a list of all waypoints called XYZ and which countries they lie in.
Press LSK next to the correct XYZ
The FMC then gives you a set of choices (straight to the wpt/intercept the track to it/if the point also forms part of the missed approach then to there etc)
Press the LSK according to your choice.
You then get the MOD FLT PLAN page where you finally get to press LSK 6R to ACTIVATE!
There probably is a sixth stage that I have forgotten!