Click on the 'Open' button, and navigate to the path of the SoundFont you wish to add. This should be /usr/share/soundfonts, if installed to the standard location specified in Section 10.2.5, “Install SoundFont Files”.

Select the additional SoundFont, then click the 'Open' button.

To change the SoundFont ID (SFID), use the 'Up' and 'Down' buttons to change the position of the SoundFonts, as desired. This does not directly change the function of FluidSynth - any SoundFont should work with any SoundFont ID number.