If you forgot to install certain libraries with Revit 2015, like I did, it is worth noting that the procedure for installing missing libraries has changed slightly. In previous releases, you would Uninstall/Change Revit through the Control Panel Programs and Features. With 2015, you now have a separate listing for Autodesk Revit Content Libraries 2015.
This Autodesk Revit Content Libraries 2015 is the one you want to select to Uninstall/Change which gets you to the familiar screen where you can select to Add or Remove Features.
From there you will be able to check any missing libraries (i.e. Canada, US Imperial, US Metric) that you may have missed during the original installation. Note that you may be asked to supply the installation media (USB) to complete this step.