Hi Jeremy,
Looks like the setQuads method on TextMarkupAnnotation is missing from the latest version.
We'll try to fix this in the next maintenance release, or in a custom build if you'd like.
In the mean time, you could try the following:
Use the "Quads" property as part of TextMarkupAnnotation, by setting it directly
e.g.
highlightAnnot.Quads = Document.GetTextPosition();