When Excel formulas are showing instead of the values, there are a couple of reasons why this may be happening and the fixes are simple.

1. If the cells’ formatting is set to “Text”, a formula will display instead of the value.
SOLUTION: Highlight the cells, right-click and select “Format Cells”, and select “General” or some other type.

2. Keystroke CTRL + ‘  (Control key and single quote key pressed simultaneously) activates the “Show Formulas” in Excel.
SOLUTION: Press  CTRL + ‘

3. “Show Formulas” button was pressed on the Ribbon tab “Formulas”.
SOLUTION: Press the “Show Formulas” button again to turn off.

4. A ‘ (single quote character) has been placed at the very first position of the formula.
SOLUTION:  remove the  ‘ (single quote character) from the formula.

Source: http://chandoo.org/wp/2010/04/12/excel-formulas-are-not-working/