The engine is now available for free here.
Read the terms and conditions, but they sound pretty good for those that just want to tinker around with it.
To get started, the official documentation can be found here.
The community developed documentation, which is well worth a browse can be foundhere.
A simple guide to get started on creating a level can be found here.
More indepth knowledge can be found in the wiki here.
Now go and enjoy!