inserting a calculated field in a pivot table.
 

I can't enter a calucated field for some reason in my pivot table. I have
about 20 or so data points populating my table and I want to calculate some
percentages with those fields but it's not creating another line. I can enter
the formula once i select insert calculated item and I can name the new line
but when i hit ok nothing is happening. When i usually enter formulas it asks
me if i want to replace the contents of the destination fieldand then
proceeds to insert another line but nothing is happening here. Could it be
that I have too many data points? Any help would be appreciated. Thanks