Problem with editing joined table

Hi,

I want to calculate a new field based on 2 other tables. I've joined the tables based on an area code and I've created a new field. But when I click on field calculator I get the following error message: "Currently it is not possible to update fields in an inner joined table that have a cardinality of one to many outside of an edit session. Please start an edit session in order to perform this operation." I've started an edit session for the table, but I keep getting the same error message. Does anybody have an idea how Ii can calculate my new field?

Thanks