Your problem must be that the cell was originally formatted as Text, so it just displays exactly what was typed into it, even a formula (like "=A1+B1"). You have been given half of the answer in a previous response; make sure you change the format of the cell(s) to General. However, after doing that, you must then select the cell(s) and use the Excel Edit|Replace function to replace all the "=" with "="; this will force the cells to recalculate and the formala(e) will work!
Regards,
Brian