Maybe this is not possible in Studio, but only in Studio Pro. Try downloading and installing Studio Pro, edit your project there and try again.
Update. It’s still now working. It won’t let me use the drop down and assign an attribute. IS there a mendix teacher or anyone that just went through the rapid development course?
Ah, as per the instruction 3 on course 8.4.2 you have to open the detail-page. Instead, you are on an overview-page where you are viewing the information. On a Detail-page you are changing the information. That is when you can add this drop-down.