Click the third button from the left in the toolbar and a dialogue with your channel list will appear. Click the "Add" button and enter the appopriate details, EG: Channel name, a key (if needed) and brief description of the channel (optional) then click "OK". This is just the previous answer in more detail.

Channel added. Just press "Add" again and repeat the process if you want to add more.

Some networks let you join one and all channels consecutively while others have limits anywhere from 25 down to 5 channels that you can be on at once. The limit is usually in place to avoid flood-kills each time a connection retries.