See the dengine.net Add-ons Repository.

The majority of the packages made available here can be installed via any of the following methods.

1. Download your chosen addon package(s) to some temporary location e.g., your desktop
2. Launch Snowberry
3. Navigate to the Addons tab
4. Click the '+' button to open the Addon Installer wizard
5. Click the '…' button to open the folder browser
6. Use the folder browser to locate the folder where you downloaded the package(s) to
7. Click Ok
8. The Addon Installer should now feature a list of all compatible packages in the chosen folder. Click to select each package that you would like to install
9. Click Install

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).

1. Download the chosen package(s) directly into the Addons subdirectory of your Snowberry installation e.g., c:/program files/doomsday/snowberry/addons/

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.