trying to use onboard synth

[zbcustom73]

just loaded reaper onto my intel imac 10.5.5.

trying to understand what I need to do to play a software synth (I have no external midi device on this mac) like the Apple Quicktime sounds? or reasynth

I have the IAC bus showing up as a MIDI output option and enabled

I have inserted a virtual instrument track and chosen the AUi: Apple: DLS Music Device
when I choose it i get an error "Build ROuting Errors could not get channel info for effects(are they multichannel?) and an OK button- I click OK and I get a window showing parameters and a comment field- one of the parameters is QuickTime music Synthesizer
upon closing that window my track has a green light in the FX slot

do I need to change my MIDI hardware output in the i/o window?

what might I be missing?