So you want to connect to our IRC channel, do ya?
Click here (yes, it's that simple!)
Enter in your nickname and voilà!
If you already have a client installed click here
Or if you want life to be slightly more complicated...
Restart Firefox and open Chatzilla (in the tools menu):
Open the preferences menu and change your username (make sure you're under global settings unless you know what you're doing):
Press OK, then type in /server freenode and press enter:
You're now connected to to the server, but have to join the channel.
Join the channel by typing /join #l4d-mappers and press enter
You're in! Now right click on the #l4d-mappers and click "Open this channel at startup" so every time you open Chatzilla, it automatically connects you