If you installed the latest version of Drift... it's for 1.4.x series of XenForo. You need to grab the version (1.3.x) latest for the version of XF you are running.
Also, just so you know, the later versions of Drift no longer use UI.X as a base... UI.X base is included in it so you basically install Drift and then make a child style for your customizations with Drift as the parent.