7538c605_Data_Mining___SQLServerCentral__Cause_Solution
[Error] 1. New Measures Aren’t Visible in the Mining Structure Add A Column Dialog Box - You can’t add calculated measures to an existing mining structure in BIDS because they’re not visible in the Add Column dialog box. [Cause/Solution] 
All I had to do was process the cube, close the project then reopen it and the new measures appeared when using the Add Column button.
An Informal Compendium of SSAS Errors, Part 2: Data Mining - SQLServerCentral
Cause/Solution