Wow.
1. When a Yes or No property is set to "Yes," the expression to compare it to is "Y," not "Yes." I hadn't gotten around to researching that particular question yet...
2. The nested brackets ( "{ DOCPROPERTY "_Instructor" } aren't just any curly braces -- they have to be Word's magic field brackets created using
Insert -> Field.
So. In case this will be of use to anyone else, here's the way to display conditional text using document properties and field codes:
{ IF { DOCPROPERTY "_Instructor" } = "Y" "(Instructor)" " " \* MERGEFORMAT }
where all the brackets are magic and all the spaces are essential.

(I feel like Garrison Keilor...)
Thanks for a forum where I could puzzle this out myself!
Caroline