Hi,
I develope on Unity3D Free version, but… Today, I get information about Ludum Dare Pro version. That’s a big opportunity to try the Pro features for me.
Here is that announcement https://unity3d.com/gamejams/LD31
Click the button on the page you linked to get a key, then try going into Unity editor, clicking Help>Manage Licence and hitting the “Activate new license” button.