Adding nodes??

How could I go about doing this I’d look on fpsbanana but the search option is broken

Check the Valve Developer’s Community. Best source on all things Hammer imo.

Your question is very vague.

Noding and properly setting up the AI can be quite complex.

This is for HL2/Ep1/Ep2/GMod

This is for CSS/L4D/L4D2:

What’s the deal with CS:S?

I tried a 2 day trial before (thought it was crap) and as far I could see, it’s just a multiplayer game.

Where do the NPCs come into it?

Bots. L4D uses the same nav mesh system as CSS.
The nav mesh system is more suited to using bots to replicate human players, since valve wrote a program to automatically generate the meshes and sample things like camping/sniping/crawlspaces and is suited to the bots going everywhere in the map, whilst the HL2 AI is a lot more confined by the mapper and as such you can tell where they will be and what you want them to do and how they should react.