If you're an Unreal 4 Licensee, you can check out the current state of my plugin on its pull request page here:
Given that it's still in development, there's no usage information formally documented yet. Here's a brief rundown:
There's a module that lives in \ThirdParty that contains the Leap DLLs and a plugin module in \Plugins that actually provides the support for the engine. You'll need to copy the dlls from the ThirdParty folder to the binaries folder, then enable support for the plugin in the editor. Once that's done you can add a LeapInputComponent to any actor blueprint you want to be able to respond to Leap events like a new frame or gesture, and then use the 'new frame received' blueprint event to write your event handler.