You can manually add a diagnosis from the "Medical Record" tab or you can do so while creating a new order.

To add a new diagnosis code, once you are in the Medical Record section of the patient's file, you will see diagnosis field that says "Add Diagnosis". 

Click on that line and enter the code required. 

Once you select the DX code, be sure to click on the blue plus sign to the right of it, to add it to the record.

Be sure to select "Save" on the bottom right once complete