How do you do that again?
When I jump on a mech to enter it it just pushes me away.
You need that energy gun to stun mechs with pilots in them to take them over, so if you don't have that find an empty parked mech. I think most of the bases on the new island have a couple of empty ones.
You also get the ability to call in a mech but I'm guessing you haven't unlocked that yet.