I have a fairly simple question.
I am trying to generate a quick, descending sound, but I want it to start on the note that I am playing. So...on the PEG, which part of the waveform is the actual note? Is it the Hold level, is it the Attack level, or is it the Sustain level? The PEG values can be set from 0 to 127. Do those values correspond to any pitch or fraction of a pitch? I am looking for something like a fifth in about 1 to 2 seconds. I am having starting on the correct note as well as getting something smooth, without a sudden big drop in pitch.

Any tips out there?

None of the above, really.  The PEG controls the CHANGE in pitch, not the pitch itself.  The Pitch is still selected by the note you play.  The PEG “Modifies” that selected pitch according to the parameters you set.
So, if you set attack to 0, it will modify the pitch at attack.  Some number greater than 0 will change how long it takes for the Attack to “Ramp Up.”

If you set HOLD TIME to a long number, there won’t be any PEG affect for a long time. 

I’m GUESSING (since I don’t have my mo at my fingertips) you could accomplish what you want with these parameters:

(I’m assuming you want the FALL to happen whether or not you release the key.)

Hold Time: 0
Attack Time: 0
Decay Time 1:  (Vary this according to how fast you want.)
Decay Time 2: 0
Attack Level: 0
Decay 1 Level: (Vary this and the next parameter according to how FAR you want the note to fall.)
Depth:  This should be a negative number, if you want a FALL instead of a rise.

You probably need to set the EG (Envelope Generator) to somewhat matching parameters so that the source waveform stops sounding after the appropriate duration.

Something like:

Attack: 0 (Instant on)
Decay Time: (Vary this to duration)
Sustain: 0 (You want NO sound even if the key is still held down)
Release: 0

...Something like that… :) Hope this helps.

Play with these parameters using a simple, stable, long-running voice like a SinePad (PRE4/64) (with Portamento OFF so it doesn’t confuse matters)

Thank you for your help.
Your suggestions worked very well. My editor (MidiQuest) was confusing me because it has the levels going from 0 to 255 instead of -127 to 127. Now that I know that a MidiQuest 128 is a Yamaha zero, I can get the pitch where I need it to be.