Add grid lines to frame your opening, and then use Edit areas>Divide areas and mesh by gridlines, then select the meshed panel(s) where you want the opening and press Delete key on your keyboard. That mesh by gridline option works great for rectangular openings in order to mesh precisely where you want. For circular openings, I've seen a tutorial somewhere on the CSI Knowledge base wiki
Here is the wiki article that stressed was referring to.
Stressed, can you contact me directly at rogersbr at yahoo dot com. I have a question for you that is not appropriate to be posted publically on the forum and I don't see a way to send a private message.