Trying to help, but I am confused. You said that you attached both cables to the twist grip, then you go on to say that you do not see where to attach the closing cable. Seems like a contadiction to me, or I'm misreading something here.
After removing the aft portion of the Twist Grip Guide you should be able to rotate the grip so that both ot the guide cable nipple holes are visible. The upper one will be the opening one, the lower is the closing one.
While you had things apart, did you verify that the throttle body cam rotates freely? Speaking of the cam, the closing cable should installed at the top.