OK, that should make answering this question easier but, I now realize I'm not sure if you want to put the iframe on the menu (hard to imagine why, so if this is the case please give as many details as possible about what you envision it doing, how you would like it to work) or, more likely, you just want the menu to load its links or some of them in (an) iframe(s). This second idea should be no problem, let me have a quick look at the code. OK, I'm back. Here is a typical entry with a target. In this entry the target is '_new' for a new window. You could make it something like 'main'. Entry example from demo code in .zip file for this script:
Then have your iframe have a name:
ssmItems=["Message Forum", "http://www.codingforums.com", "_new"]
So, whatever name you give your iframe, use that as the target value in an entry and it will open in that iframe when clicked.
<iframe src="whatever.htm" name="main"></iframe>