I had a better time using the Java sounbank until version 1.1 (linux). Since 1.2 it seems that it isn't possible to use it anymore.
I just can set 'Gervill' or 'Real Time Sequencer'.
So far I am using Timidity, but there is no good soundfont at all.
Since this configuration depends on the Plugin (Java Sound Api plugin) that provides the MIDI Device, this configuration was moved to plugin's setting.
See at Tools -> Plugins, select Java Sound Api plugin and you'll able to configure a custom soundbank.