Microsoft announced that the Office for Mac 2011 (and Office 2008 for Mac) is ready for Apple's latest OS X Mountain Lion. But the only problem remaining is the inability to manually download and install future updates because of Mountain Lion's new feature called Gatekeeper.
Gatekeeper protect users by allowing you to choose to only download and install software from known developers but due to its architecture, updates will not run when you download and initiate them on your desktop.
To keep your Office for Mac updated, Microsoft recommends that you use Microsoft AutoUpdate.
Source:- Office for Mac