Two ways...
1. Right click the cell. Select Format Cell -> Protection -> Uncheck Locked
Cell
2. Select Tools -> Customize -> Commands tab -> Format from the list on the
left. Scroll down the list on the right to until you find a Lock Icon. Drag
that icon onto one of yoru existing toolbars.
Number 2 is my prefered method. When you sleect a cell it will show you
whether the cell is locked or not which maked things a lot easier...