Honestly the constraints are fairly independent of Playmaker. Setting them properly is a big part of their use. They are automatic.
In Playmaker you will be managing the character's moves around the disc but that's it. The constraints are there to simulate a gravity that always points towards the center of the disc.
You must apply the constraint components to the character but also use the disc as a reference in the proper fields. Try the Look At one for starts. Making tests is the key, no time is wasted as it teaches you Unity empirically.
Adding 2D colliders, like on the disc, can be used to block the character from ever passing through the disc but I suppose this could also be done by fixing a vertical distance.
You can drag a component into a state in Playmaker's Graph View, it will open a window to set or get the component, so you can change the component's values as you see fit.
If you use this way of controlling Unity components, don't forget that you'll need to run the Linker Wizard (look it up on forums) before publishing your product.
Otherwise, look into how to code simple actions by using Unity's script API manual. The best way to do that is to generally copy a simple action, see how the code does what it does and then try to modify it slightly to create a new action that can modify Constraint components' values.
