You can install pistonpusher like most other add-ons. Open blender, go to preferences, click on addons, click on 'install...' and select the pistonpusherxxx.zip where you downloaded it. Don't forget to activate it. You do not need to set an assets path anymore but it's highly recommend to prevent data loss during updates.
There is also a k- and decal(machine) pack, see the Kit ops/Decal Machine documentations on how to install them.
If replacing the files fails for some reason:
First remove the add-on in blender and then restart the client. Now you should be able to re-install the add-on safely.
All features are accessible from the pistonpusher pop up menu. The default shortcut is SHIFT+E.(Can be changed in keymaps, search for 'pistonpusher')
To append/instantiate a piston, you simply select a thumbnail. New instances are always 'reset' to a sort of default expected state. This means that the B empty will always be re-parented to its host armature even if the last instance was already placed and constrained.
Depending on object selection, a piston will be placed at an object's location, the 3d cursor or the snap tool/modal will be started.