Hi,
1: Create a github account
2: Get sourcetree or github Client app
3: Clone the Unity 5 dedicated custom action rep:
https://github.com/PlayMakerEcosystem/PlayMakerCustomActions_U5 on your computer ( this will be a local copy of that rep)
4: Open that local copy with Unity 5.6
5: Install PlayMaker ( you have to manually install it because of the errors you'll get, the install package is inside PlayMaker/Editor/Install )
6: Add your action inside Assets/PlayMaker Custom Actions/Audio/
7: verify that Unity compiles, verify your actions works as expected
8: in source tree or github client, commit and push that new action.
Let me know if you get stuck in one of these steps.
Bye,
Jean