Hi Rinelle – I just tried out Mantra on a test site and the menus seemed to work fine. Are you doing yours per:
http://codex.wordpress.org/Appearance_Menus_Screen
If so and it’s not working, try deactivating your plugins just in case one might be troublesome.
Yep. That’s how I’m doing them. I can set up the main menu just fine, but when I try to put in a new menu item, and I intent it (as I’m assuming you do to get it to be a drop down menu option, not just another main menu category?), it appears to work, then when I save the menu, it removes the indent, and turns the item into a main menu item.
I tried deactivating and reactiving all of my plugins, but it did the same thing.