You can activate UberMenu on individual sites within your MultiSite installation.

Each individual site within the MultiSite network is considered a single installation relative to the CodeCanyon (single-use) Regular License. If you want to use UberMenu on a WordPress MultiSite installation, you must purchase a license for each site on which it is activated.

That being said, UberMenu is not tested network-activated with WPMS (since the license does not cover this), and the timthumb image-resize functionality of UberMenu may not work properly in WPMS installations (so you may not be able to make use of the timthumb automatic resizing of thumbnails, and need to upload your images already pre-sized).

UberMenu will not work globally (network-wide) on WPMS – i.e. you can’t automatically have the same menu on every site. Each site will have its own independent instance of UberMenu running.

Possible Exception / Grey Area

If you are running a site with multiple subsites that is truly a single site, where you are using an identical menu on each subsite, I see this as a grey area; technically the Regular License does not cover multi-site use, but in this case I understand the argument that it is still single-use. In situations like this (a true single site with identical menus on each subsite), I leave it to your best judgment to determine whether your usage constitues single-use and is covered by the Regular License. Do what you think is right. If you have any concerns, the best thing to do is contact Envato Support for official clarification.