I assume you already have that field in the PivotTable?
Can you check the PivotTable options dialog and make sure under the Display tab that "Show calculated members from OLAP server" is checked. (It should have done that for you when you added a calculated dimension member.)
When you open up OLAP PivotTable Extensions and look in the Calculation Name dropdown, do you see your calculated member? And does it still have the name you entered?
When you find the field in the field list and mouse over it and choose click the arrow, do you see that member in the list of members in that dropdown?