Seems that a lot of people have embraced this upgrade but have not read it very carefully. It is not a PtL upgrade.
Experimental Interface says the following: Once per round, after you perform an action, you may perform 1 free action from an equipped Upgrade card with the "Action:" header. Then receive 1 stress token.
So in order to use it, you need to have an equipped card that has an Action on it.
Currently there are only 9 cards that this works with:
Expert Handling EPT
Squad Leader EPT