I have a Laser Pistol, a Laser Pistol Focus Optics and a Laser Pistol Recycler in my inventory but none of them are available when I press the 'mod' key in the inventory for the Laser Pistol. How do I use these items?

You need a Laser Pistol(GRA). One can be bought from what's his name in Goodsprings (the trading post owner)
A regular laser pistol cannot be modded

Okay, thanks a plenty.