When attempting to use my Inventory i get this error:

rigidbody.velocity assign attempt for 'vShooterMeleeControllerRPG' is not valid. Input velocity is { NaN, 0.000000, NaN }.

UnityEngine.Rigidbody:set_velocity(Vector3)

Also is there a further explanation for how to implement the use of mana for actions?
i want to start implementing generic actions for magic.

gday mate, the rigidbody issue is on the invector controller side, check their forums as they have a hotfix for it.

as for mana for actions, the mana system is based off the health/stamina system for the controller. You'll need to reference the mana controller script on the player and simply call the add/remove mana functions as desired. My spell system uses weapons instead of actions, so I haven't done much playing with generic actions but you might be able to other call directly from the action or use a bridge script to pass calls between the two.