Where is the key to gain access to the Forbidden Zone Dome Entrance?

All i have left to do is complete Field Research, but i can't, because somehow i have missed the key...and the Vault is currently no help at all, giving a description of the interior but not mentioning the access key at all.

http://game.downloadatoz.com/tutorial,25980,easy-way-to-get-into-forbidden-zone-dome-in-fallout-new-vegas-old-world-blues.html

You might also want to consult with http://www.gamefront.com/fallout-new-vegas-old-world-blues-dlc-walkthrough/

EDIT: Like said below, you need to return the tech to the think tank in order for the Forbidden Dome Zone to unlock.

Refer to the links above for more info.

It's unlocked through the main quest. There's no actual key.