You turn on all five power switches around the map and go through the portal at the central area.
Power Switch Locations:
Spawn: In the small room to the left of where you spawn.
Journey into Space: In the tunnel connecting the Rocket Trap with the Bumper Cars.
Kepler System: On the side of the Moonlight Cafe bar, next to a possible spawn for the boom box for the EE.
Astrocade: On a wall to the left of the wall containing the teleporter.
Polar Peaks: Across from Quickies, near the entrance to the rollercoaster.
Once all five are on and you travel through the four portals (excluding the central one), the central one will open the way to Pack-a-Punch.
Like I said, you'll have to do the map's easter egg quest.
You have to complete the whole easter egg first. After the UFO is destroyed, it should spawn directly in front of the teleporter to Pack-a-Punch not to long after Willard finishes his quote.