UIGrid is for run-time positioning. I don't recommend using it at edit-time unless you have to.
Instead, update to 3.0.7. Create a UIWidget covering the area you want each of your "cells" to be. Inside of it add your button. Use the new anchoring system if you want to make it relative to other buttons, or just drag the widget in the scene view. As you duplicate the widget with the button inside and drag it around, each will snap to its siblings automatically.