Joining Nodes Help Needed

Hey all,
i've been working trying to figure this one out, and i'm getting pretty annoyed that I can't..

The long and short of it is.. I'm looking for a way to take a shape made from lines and fill it.

The bit I need is how to join the nodes, i've been trying to use joinwith command (with combined and not combined lines) but i'm either having real trouble with the syntax or it doesnt work how I think it does.

I've recorded a macro doing it, but I don't understand the commands it uses.
Can someone give me a quick hand here,
Thanks alot!