These addon files are the ones that were available at the 3drad.com/addons page, as well as newer addons published by members of the community.
Most of these addons are downloaded in the format of an installer [from the 3DRAD website - not repackaged installers], making them quick and easy to install. Simply run the installer and it will automatically install the addon into the 3DRAD folder. Others come in a ZIP archive, which will need to be extracted to your 3DRAD folder. Please see the included readme file on how to install those addons.