The following methods will all take you to the FUMBBL chat.

Connect using an IRC client

This is the preferred method as the other alternatives are rather buggy. Set up your favourite IRC client to connect to:
Server: irc.fumbbl.com
Port: 6667
Channel: #FUMBBL

If you have a client installed, you might be able to connect to the chat right from this page.

Mibbit - Web based IRC client

If you don't have an IRC client you can connetc using Mibbit:
Mibbit