Modified 11 December 2021
See here how to get your eMondrian XMLA service address.
In this example an address is
http://192.168.100.4:8080/emondrian/xmla
Open Tableau Desktop, select the Data menu item, then New Data Source. Select To a Server -> More... -> Microsoft Analysis Services.
Paste address of Mondrian XMLA service as Server and click Sign In.
Then choose a cube and go to Worksheet.
A Worksheet with cube data will appear.