You can add new settings for any
network/channel by clicking on the 
button.
You can then add the network for your settings to take place (or check the "Global network settings" box to indicate that those settings will be available for all networks). Then add the channel you wish to add settings for (or check the "Global channel settings" box to indicate that those settings will be available for all channels) and finally click on the "Confirm" button. You cannot check both "Global network settings" and "Global channel settings" because these global settings are already represented by default, as the "All networks - All channels" option, in the main dialog's s "Network - Channel" selection box.
To delete a network/channel entry, simply select it from the
list and click on the 
button.
To entirely clear the network/channel list, click on the 
button. An option to confirm your
selection will popup. After you select "Yes" the list
will clear.