You can get a summary at the Library Summary page.
The main class hierarchy to dig begins with the abstract Control class; the most important one is the Frame, which is a window control. The registry is encapsulated in the Registry singleton. Once installed, use "import net.BurtonRadons.dig.main;" to get the main controls; use "import net.BurtonRadons.dig.gl;" to get the CanvasGL class and the many components of that.
Compile and install dig by running the "go.bat" program in the install directory.
If there is a complaint with an error message like "c:\dmd\bin\..\..\bin", then you need to reinstall DMD as there was an incorrect sc.ini version. Likewise, if you get a complaint from lib.exe about not providing a library, unzip dmdalpha.zip again to overwrite the incorrect version in dmc.zip.
Once it's installed, it'll automatically build and run one of the example programs. The examples directory contains other stuff to play with.