Currently, dimensions cannot be used with calculated fields when connected to a cube database. Instead, I'd recommend using a calculated member:
also, you may find this article useful:
Can we create a calculated member to create a new field which will have a new format of a particular dimension. (ex: If year dimension has years in the format FY16 I want to change it to 2016 so that I can create a line graph). And do we have to write an MDX statement to achieve this in calculated members?
I highly recommend not connecting to essbase cube directly via tableau connector. Too many limitations and issues with this approach.
Instead extract your data out of cube via some ETL and then load into tableau.
There is more than one way to build the ETL. Here some discussion on the subject: