This works for me in version 3 of the menu. However, in the 2 versions of menu 5 I've tried it gives me an error "Object expected". However, it does actually call the menu up and display it properly in both IE 6 and Netscape 7. It's just a matter of making the error message go away, as it appears to work fine.
Here is the line I use to call up a hidden menu:
<A href=# onMouseOver=popup("mainmenu2") onMouseOut=popdn()>Menu</a>
This is the line that gives me the error message, line 8, character 1, Object expected.