Now that you have chosen the pin on which you would like to assign a function, you'll need to use the contextual menu to choose the proper function to be assigned.
The menu that will be shown contains some direction sub-menus, then the possible functions accessible from the selected pin.
Figure 1. Assign a function to a pin
The result will show the assigned pin, with its default name.
If you want to change the pin configuration, you can choose another function, using again the same assignment operation.
Removing the assigned function is done with the same kind of operation, with the 'not used' option.
Figure 2. Unassign a function from a pin
Both textual editor and graphical editor are sharing the same resource (the .gpio file).
Please note that for the assigned function to be visible inside the textual editor, you will need to save the current graphical editor.
NB: Saving it (using the floppy disk from the toolbar icon) is sometimes not available. If this occurs, you'll need to click on the graphical editor tab in the editor area, then the save icon will be accessible.