Go to the Steam workshop page for Synergy and subscribe to the mods/addons you want.

Now that you have subscribed to everything you wanted, navigate to this path on your PC: \steamapps\workshop\content\17520

The full path is usually like this: C:\Program Files (x86)\Steam\steamapps\workshop\content\17520

There should be a bunch of folders and their names are only a bunch numbers. Copy all of these, because these are all the mods/addons you have subscribed to in the Synergy workshop page.

Next you need to go to your Synergy game folder. The path should look like this:
C:\Program Files (x86)\Steam\steamapps\common\Synergy\synergy

From here you should find a folder called "custom", if its not there you should just be able to create one yourself and it should work. Paste all the copied folders in that "custom" folder.

Your addons/mods should now work!

If they don't, your mod/addon might be outdated and won't work with the new versions of the game.