In Spotfire, you cannot join on a calculated column.  The calculated column will not appear in the list of columns in the Insert Columns dialog.  This limitation is by design, as calculated columns can change during the course of the analysis, which could change the rows to be returned.  The way to get around this is create the calculated column as part of a transformation, as transformations only occur once when the DXP is opened and therefore, can't change the way regular calculated columns can.  In order to use a calculated column in a join, create the calculation as part of a transformation (Insert, Transformations, Calculated column).