The home side looked solid again defensively without really threatening and once Noe Pamarot headed the opening goal just before the hour there looked to be only one winner.
Robbie Keane and Mido completed the scoring in the 3-0 win to set up a quarter-final against Newcastle United at St James Park on Sunday March 13.
"We never made it a cup tie," said Megson.
"We made it a little bit too easy for them. The first two goals came from corners and they were not exactly towering headers.
"We didn't really have people who could raise the level and we struggled."
Forest boss Gary Megson was disappointed with his side's performance as we won Wednesday night's FA Cup fifth round replay at the City Ground.