Whenever an Input Channel needs to be selected, for example when manually attaching a channel from an Input Plugin to a Virtual Console widget, the Select Input Channel dialog is used.
The dialog is very straightforward; there is a list of input universes and each universe that has an Input Line attached to it, displays the line's (or the attached device's) name after the universe number. Under each operational universe, there is a list of channels provided by that universe. You can choose from these channels or select one manually.
If you have an Input Profile attached to the universe, you see the individual channel names and numbers for the device. If there isn't a Profile attached to an input universe, you will need to type the channel number manually, by double-clicking the item that tells you to do that and then type in the channel number manually. See also the tutorial on input profiles
NOTE: All Virtual Console widgets that have been made external-input-aware, provide an option for automatically detecting the input channel as well, so that you don't need do this selection manually.