There are two ways to add an attachment. 

Option 1: Add an attachment for someone that you have tagged

Go to the case of the student and click on notes (or scroll down to the notes section).

Type on the Add a Note section and you will see a new box that has the option to attach a document.

Select the document you want to attach and click on Upload and then Save. 

Option 2: Add an attachment in general 

Go to the menu bar and select Attachment (or scroll down to the Attachment section).

Select the type of attachment you want to add and click on it.

Search for the document you wish to attach by dropping the document on that box or clicking on it and search on your computer.

Click on Attach and the attachment will be now in that section.