See the dengine.net Add-ons Repository.
The majority of the packages made available here can be installed via any of the following methods.
At this point the Frontend will install the selected packages and make them available for you to use via the Addons tab. You can now delete the package(s) from their temporary location (e.g., your desktop).
Once installed you still need to “enable” packages via the Addons tab, by checking the box(es) adjacent to the name(s) of the package(s) you wish to use.
If for whatever reason you do not use the Doomsday Engine Frontend, you will need to download your chosen package(s) and then install them manually. Typically, each package contains it's own installation instructions which must be followed for a successful manual install.