I'm having the same issue as someone who posted a month ago, but the fix doesn't work for me, since I don't have control over the php version on my server.
The errors says: "Warning: Creating default object from empty value in [...] wpm-tree.php on line 182" and "Warning: Creating default object from empty value in [...] wpm-menu.php on line 125." The menu finally shows up (and seems to work) beneath a long list of these error messages.
Using menubar 5.0, wordpress 3.5, php 5.4.9.
The problem just showed up yesterday, but I haven't updated anything in over a week. So it must be something server-side, right?
Anyway, any workarounds that anyone can think of, besides rolling back php?