I have already tryed to do this, add content us imperial content to my revit 2012 mep software 2 times from this option below. Uninstall or change a program, select the Revit application, and click Uninstall/Change.
The first thing to do would be to verify if the Mechanical-Default.rte file is located in the folder specified (C:\ProgramData\Autodesk\RME 2012 Templates\US Imperial). If the file exists in the folder, verify if you can open it directly.
If the file is missing from that location, you could also try doing a Repair on the installation (through the Control Panel).