No matter what I select or where I look I can't find the ruler anywhere. I'm using Blender 2.74, what am I doing wrong?
Open the toolbar in the 3D Viewport using T. Navigate to the Grease Pencil section and click the 'Ruler/Protractor' button. You can now click and drag in the 3D Viewport to create a ruler and or protractor measurement.
For those landing here after Blender 2.8 release:
Plus: nice video (not mine) introducing how to use it: https://www.youtube.com/watch?v=OZECjQS5yVo