Normally, you can drop your dimension on the sheet and Tableau automatically assigns them shapes. You can change shapes by clicking Edit Shapes in the marks pane. Hope this helps.
P.S: It is often a best practice to post packaged workbooks with your question, you get faster replies !
In Marks, select Shapes instead of Automatics --that usually are dots--, as a result Shape will be displayed in Marks.
Then drag your dimension Type over Shape on Marks shelf.
As Manas Ranjan Kar mentioned, it is easier for us to help if you upload a workbook with sample data.
I hope this helps
Thanks Ramon, That was the step I was missing! As soon as I selected Shape for the Mark instead of Automatic, the Shape button displayed, and I was able to drag the Type dimension onto the button to get the shapes to change based on that dimension.
I'm glad you have a solution.