go to the gym then go to the tube farthest away from the entrance go behind it hit a the go give the thing you got to the boss at the power plant. It worked for me so it should work for you.
Power Plant
Restore the power plant and find the team rocket dude
you have to have completed the power plant mission and he will be outside the power plant on the left
See below
The grass in Kanto by the Power Plant.
it is i the kanto region in soul silver after you defeat the elite 4
I don't know when but after a while the power plant
Battle Misty
At the Power Plant.
Go and talk to the gentleman at lavender town's radio tower.
start by heading north and taking route 10 to the power plant
it's in the gym where u vs. misty