Right-click a 'track info' area and you'll be allowed to select a MIDI port as an option.
There are 16 softsynth ports, they are at the bottom of the list for ports (17 - 32)
If you select an unassigned port, RB asks you to pick a VSTi/DXi synth (like GPO) to assign to that port.
Or you can select a port with a synth already assigned.
In RB you get 16 hardware and 16 software ports to use.
Hardware = 1-16
Software = 17-32


I do not work here, but the benefits are still awesome
Make your sound your own!