Try this; put the tracks in CLIPS view(Regions for 9 and older versions of PT), highlight the clip on the midi track(the whole thing) and copy, then move(with the p and ; keys) to the Instrument track and then paste. If that doesn't play anything, go to the MIX window, select View>Mix Window Shows, and check Instruments. This shows a row across the top of the MIX window(with info only at the top of Instrument tracks) and set that to your SD2 plugin.
just wondering if you are not managing to link the midi to the instrument plugin, and scrapping the midi track, and placing the midi on the instrument track itself, is more efficient in the long run
. Once the midi notes are on the Instrument track, the midi track can be deleted, or hidden if you want to save the midi info for some reason.