Want L4D 1 and 2 models in GMod, but can't

Just as the title says, I want to play around with Left 4 Dead 1 and 2 characters in Garry’s Mod, but for some reason I can’t. In choosing which games for Garry’s Mod, the names for L4D just aren’t there. Example: http://tinypic.com/r/vzwioo/6

I bought all of the games through Steam. Is there an updated version I need to download, or do I need to buy it again? I bought Garry’s Mod specifically to play around with the L4D models. This makes me sad.

Edit: Okay, Mr. I’m-going-to-vote-this-dumb, instead of being a “higher than thou” jerk, how about some feedback or help? I already looked at the sticky and I’m SO sorry for not saying so and that I didn’t quite understand it.

Please help. I feel as though I wasted money on this game.

You can’t mount L4D content in gmod like hl2.
But what you can do is manually extract the models and materials from the l4d’s vpk’s and put them in gmod.

Thank you for the responding.

I’ll try to figure out doing that, but did you mean to find the models and materials and put them into the addons folder, or somewhere else?

I’m not dumb, I swear- this is some pretty complicated stuff for a person new to GMod… And to have a couple people basically say someone’s dumb for asking for help… well… are most people at this forum like this?

Step-by-step guide for adding L4D models to Gmod

This is like the twelfth time I’ve linked to this tutorial this week. :S

Thank you. I’ll give it my best.