Download the example files here: Estimating_Joint_Reaction_Loads.zip
Matt DeMers gave a webinar demonstrating how to use OpenSim to estimate Joint Reaction Loads.
You can find a link to watch the webinar here:
http://opensim.stanford.edu/support/event_details.html?id=13&title=Webinar-Estimating-Joint-Loads-in-OpenSim