This problem has driven me nuts on more than one occassion, so I am documenting the solution here for reference. Maybe this will help you.
Problem: Enter formula into cell such as "=B2". After entering formula, instead of seeing the value of the B2 cell, you see "=B2".
Step 1: Go to "Tools" menu, select "Options...", then select the "View" tab. Under the "Window options" section, is "Formulas" checked? If so, uncheck it, click "OK", and hopefully you're done. If that doesn't fix the problem, or if "Formulas" is unchecked, then go to step 2.
Step 2: Change the number format of the cell to "General". If this doesn't fix it, then copy the text of the formula, paste the text in Notepad, delete the cell, change the format of the cell to "General", then paste the text from Notepad into the cell. That should fix it.