Or, for example, Helvetica appears in the dropdown font menu, but not Helvetica Light.
Okay finally figured it out, at least for a Mac. Go to Format > Character. That will take you to a window that has the font “Family” name (e.g., “Avenir”) in the left hand column. Then you can choose a “Typeface” (e.g., “Book” or “Light” or “Regular” or “Heavy”) in the middle column. What I couldn’t figure out before was where to choose the typeface. Not all the typefaces for a font family show up (e.g., Avenir Roman does not show up, even though I have it on my Mac), but most of them do.