Jeff,
I found this explanation online, hope it helps.
First of all, you need to change PivotTable options to enable the display of calculated members (check PivotTable Tools / Options / Options / Display / Show calculated members from OLAP server). When you do that, you choose to get all calculated members available from a dimension/attribute. Now, if you have 100 calculated members into an attribute, chances are that you want to select only one or two of those members. Unfortunately, Excel 2007 shows you a list of disabled checkbox that cannot be used to select only the members you want.
I found only one workaround to this: use the "Convert to formulas" command into OLAP Tools menu and then delete all unwanted members.
I understand the reason for this behavior: Excel 2007 generates a MDX query using the AddCalculatedMembers function, that returns all calculated members.
Justin