First, activate the Pack-a-Punch machine following these three steps:
1. After you've turned on the power, head to the generators room in the bunker and interact with the red computer screen. A lock-down will be activated, spawning a horde of zombies and Nova-6 crawlers. Survive this horde for one minute and the lock down will be over.
2. Go to the Center Street and find the four houses with this green Nova gas leaking out of them.
3. Find the ventilation units and press the interact button to repair
After following these steps, the blast shield will open up and you will be able to Pack-a-Punch in the bunker.
Now head back to the Rushmore AI for the next step