I see you are not a Beta user so you do not have global variables... So you will have to set a local variable and then use Set FSM Int to update the Bullets variable where it is located..
I think that the new version 1.2 will be out soon and then you can take advantage of Global Variables which will make more sense with this Action..
But if this Action is in the same FSM as the Bullets variable then it should work fine.
Q