Plug-ins for Outwiker can be found at here.
In order to install a plug-in one needs to download the archive with plug-in's files and unpack it to the plugins subdirectory located in the same directory as "Outwiker.exe" (for portable version) or to the outwiker\plugins subdirectory of the user's settings directory (it is C:\Users\USERNAME\AppData\Roaming\outwiker\plugins under Windows 7/10 and /home/USERNAME/.config/outwiker/plugins under Linux). The easiest way to open the plug-in folder is to use "Help / Open Plugins Folder" menu.
To post a plug-in written by third party on the OutWiker's webstite, please, submit it to jenyay.ilin @ gmail.com with detailed description.