With the TTS1 Synth Rack open (so you can see the 16 sliders etc) left click where it says 'Piano 1' next to the slider
Click Preset, Guitar, then you should see Mandolin in the list
(Or you can use the patch browser button for the midi track and find it that way)
Also, DimensionPro has a 'Bouzouki' patch (not sure how good it is) - but a Bouzouki is basically a large mandolin, so playing that in a higher register would probably be very similar....
Hope that helps!