Do one thing, on the Variable Editor box, first you put the name of the variable, then you select its datatype which in your case is 'Detail' then after that it says 'Associated Dimension'. It basically is asking you What is the associated attribute to which it should be a detail of? On the right of that there is a small browse button. Just click on it. And choose the object in your case will be Unique ID that you merged from the first query. Let me know. I can't give you the screenshot right now. If you cannot find it, let me know, and I will send it to you tomorrow.