Note that the distinctions between Unity "free" (now called "Personal") and Pro have changed with Unity 5. Both versions support plugins now, so if you can upgrade to Unity 5, this issue is moot. Well as soon as we update the assets themselves; the SDK released Unity 5 compatibility a couple of days ago. In fact, if you feel adventurous, you can grab the develop branch, which contains Unity 5 support, from here: https://github.com/leapmotion-examples/LeapMotionCoreAssets