Helpme undestand MDX exists function

After spend almost decade in OLTP world I got the courage to try to build my first CUBE. I installed VS2012 and created Business Intelligence -> Analysis Services. I did build Data Source Views, Cube and few Dimensions. Everything was perfect in EXCEL until I get requests from users to do OUTER JOIN or EXISTS filtering in my cube.

I start googling and find that there is MDX Function EXISTS which cloud do the job for me.

Since this is MDX expression I have no clue where should I write that.

In cube Measures, In dimensions attributes ,Calculated Member or in Data Source View ?

Here is my business problem . My DSV looks like

*Business case is to make EXCEL users be able to filter OSR_DUG_IMV Dimensions and Attributes, measuring table NABAVA , for members that do not EXISTS at table IZLAZI. *