In versions of Number previous to the '09 version, you could only create functions and formulas using the mouse if you wanted to refer to other cells. But in Numbers '09, you can now use the keyboard (about time!).

To use the keyboard when creating functions or formulas in Numbers '09 you need to hold down the "Option" key and then use the arrow to move to the cell you want to select. If you want to select a range of cells you hold down the "Shift" key to define the range.