1. Open empty OpenStudio v2.8 model in the OSApp
2. Use the Add Interval Schedule From File measure with Apply Measure Now to add a ScheduleFixedInterval object the the model. Save the model.

The Schedule:FixedInterval appears in the .osm in a text editor, but is not listed under Schedule Fixed Intervals in the app. Saving, closing, and then re-opening the model will make the Schedule:FixedInterval appear in the app.