I agree, it should be straightforward but I am getting errors. As per the screenshots below, it is saying the field [PROCEDURE CATEGORY] is invalid, but you can see from the column header that it is the correct name of the column.
I am selecting the field after I type CONTAINS - see orange. For some reason it is picking up the full name (worksheet, file name) - screenshot 1 which it hasn't done before when I made other calculated fields. I tried both ways by taking out the excel file name, leaving just the dimension name - screenshot 2, but either way, it doesn't recognise the dimension name.
I have solved the problem, I had an obsolete calculated field called pROCEDURE CATEGORY which it was picking up instead of the dimension I was after so all fixed and working.