STEP 1 - Make sure you have GiD installed. The DEMPack Team always uses the last Developer version of GiD. If you wish to install it now, get it from GiD's Download page
STEP 2 - Download the D-DEMPack package that fits most your Operating System (Windows 32 bits, Windows 64 bits, Linux 64 bits) from The DEMPack Website (label 'Software').
STEP 3 - Extract the content of the package you downloaded and move it into the Problemtypes folder in the GiD installation folder.