Downloading and installing addons
When you've finished playing the main Dink Smallwood game, your adventure with Dink has really only just begun due to the wide collection of fan-made and official mods available for the Dink engine that may provide you with hours of interest. Typically these add-ons are called "Dink Modules/Modifications" (DMODS), or "Add-on Quests" by RTSoft. Typically Mystery Island is included by default.
 Where to find them
 What to do with them
Presumably you will download them and be able to install them by double-clicking. If you're using either Dink 1.08 or Freedink which includes DFarc, the Dink Frontend should automatically associate .dmod files and will offer to install them automatically for you. From there you should be able to start playing by selecting it in the list. If there is no file association, you may install them from the "File" menu in DFarc.
 Dink 1.09
If you're using Dink 1.09 (HD) the installation process is a little different and may be performed in a few ways. The first is to use the in-game installer which will prompt you to paste a link to the file in question in order for it to be downloaded and installed by the game engine. After this is done, you may select it from the list. Please be aware that many mods will not work properly with this version as specified on the DMOD Compatibility page.
However, if you have already downloaded the .dmod file, you obviously don't want to re-download it. To install manually you may drop the .dmod file into the folder in which dink.exe lives. On Windows this is most easily performed by going into the shortcut properties then selecting "Open file location". After dropping in your .dmod file(s), relaunch the game and it will automatically install your addons.
The third option if you're wanting to reuse your existing Dmod installs is to use the -dmodpath option to run Dink 1.09. This is performed by using the command line and requires you to specify the path of where your installed D-mods already are. For example if you have used 1.08 to install them in C:\Users\Bill\Dmods\ you could create a batch file with the contents "dink.exe -dmodpath C:\users\bill\dmods" in the folder where Dink 1.09 is installed. By running this batch script your existing D-mods will appear in the list allowing you to easily continue playing without worrying about having to copy files over.
 For everyone else
A .dmod file is a .tar.bz2 file with a different extension. If you're on a platform without DFarc, you may decompress them with your standard decompression software (such as tar) and place them in the relevant file location for the engine to find them.