You do not need the rendering library installed if you don't plan on rendering. (I suspect most Revit Structure users don't care much about rendering.)
Installing Rendering is an option in Revit Structure, when you install, make sure you check the right box in the installer.
If it's not installed, you need to re-install Revit.
If you're not going to render, just check the "don't show me this again" box in the warning dialog.
Joe