Just make another query on the crosstab you made. His function has too many brackets.
(DateDiff("m",[Stage1],Nz([Stage2],[Stage1]))
should be
DateDiff("m",[Stage1],Nz([Stage2],[Stage1]))
query 1 = crosstab
query 2 = find the time between stages with datediff (check for nulls if you have to...