Background
Older versions of IE do not support HTML5 elements.
Note that generally this is no longer an issue, as almost every WordPress theme will include the HTML5 shiv in order to support older versions of IE. If you’re still experiencing this issue, it probably makes sense to update your theme.
The Problem
Because older versions of IE cannot handle processing HTML5 elements – specifically the nav element, styles cannot be processed and rendered for those elements, and the result is an unordered, unstyled list.
Identifying the issue
Check your markup. If you’re using IE and the #megaMenu element is a nav (rather than a div), this may be your issue. You can also check your Control Panel for the HTML5 option and check if it is enabled.
The Solution
You simply need to deactivate HTML5 via the UberMenu Control Panel > Main UberMenu Configuration > Miscellaneous > Container Tag (Appearance > UberMenu > Advanced Settings > HTML5 in UberMenu 2). Otherwise, use make sure you don’t have any plugin/theme javascript errors or residual styling affecting the menu.
If you wish to use HTML5 on your site, you’ll need to use something like the HTML5 shiv to properly support HTML5 element rendering.