Designer(s) are located at the top of the library below Product Types and left of the Search Window. See below. 

Click the dropdown arrow to see all designers including the search feature to find a specific designer faster. You can select any designer or combination of designers to sort your designs.

To unselect, click the X to unselect all designer(s) or access the drop down to unselect specific designer(s). 

In the example below, Joy Kate Designs is selected.