Looks like you are running into one of the functional differences of using Tableau against a cube vs a relational database. This article describes these differences with some examples: Understanding Functional Differences Between OLAP and Relational Data Source Connections | Tableau Software .
While you can't use dimensions on your calculated fields, you can use them in calculated members. Here's How to Create a Calculated Member . Once there, you'd want to use your MDX skills!
Hope this helps!