Have you looked at the E+ example files? HP_wICSSolarCollector.idf and SolarCollectorFlatPlateWater.idf are the two that use TemperingValve (this is a link to the I/O reference guide, you need to read that too).

It needs to be put on its own branch, the other only branch would be the one with your WaterHeater:Mixed object (no bypass or anything else)