I want to activate the atomic bomb, and have insufficient Explosives skill.
How do i blow it up

take some of the drugs you find rummaging throw peoples stuff. there is one that has no negative effects and allows you to disarm it.

or you could just blow it up...

Use a trainer.I recomend downloading from http://www.cheathappens.com

You can eat some mentats, I think that increase you skill by like 9 or 10. Save before you do it so if it doesn't work you don't waste your mentats. There are a few apparel items that boost explosive skill, but the chances of you finding them before you level up are slim to none. If the mentats don't work, go out and level up and increase your explosive skill to a point that, with the mentats, you can arm the bomb.

P.S. get the bobblehead before you blow it up