Range Selection (Formulas)
Once you have select a formula from the list ([spreadsheetlivedev:Formula Menu]) you will be prompted to select the range of cells for which this formula will apply. For example, if you selected the SUM formula, you would need to define which cells to add together. This is done by clicking and dragging the desired cells. Then simply press ENTER. 
Image 1 - An example selection for a SUM equation.