Hi there Louis,
Hope you're doing well.
After right clicking on the EmployeeDetail Block and select the option Fetch Data From Database, you will see something like this:

Just click (left click) on that white space (meaning Aggregate Canvas), and a image will appear for you to select your entity:

Then you will need to select Department entity (step 2). It should appear in the list.
The Aggregate Canvas is the area in Service Studio where you can edit / manage your Aggregate.
Hope that this helps you!
Kind regards,
Rui Barradas