Recently built a Prusa Mendel i3, but as built, the LCD screen just hangs on the wire. Lame. First non-calibration print, two brackets to hold it in place!
Its pretty obvious from the picture. Two brackets required. It is symmetric so two of the same works one for each side. Use a small screw, and or predrill the hole to clean it out. I did not follow this advice, used a No 6 wood screw and no predrilling, and one bracket delaminated a bit.