It is not possible to take an extract of a multidimensional data sources. It is a limitation of cubes:
Tracy, thanks for the quick reply. This makes me so sad. Do you have any recommendations for building reports using a connection that can be made into an extract?
While Tableau can't extract cubes from SSAS, there are some third party tools and code that can create a Microsoft "offline cube" (.cub). We can connect to these, too -- and we treat 'em just like cubes.
Here's a blog that takes about the process of creating a .cub file "automagically". It's not for the faint of heart, but it might give you something to go on.