Client diagnoses are entered on their Initial Assessment & Diagnostic Codes tab within the case notes section.
A new case is created either by clicking the client's Notes button for the first time or by adding a new case manually by going to Clients > Details > Cases > (blue button on the right) + New Case.
Within a client's case you'll find the Initial Assessment & Diagnostic Codes tab. Click the + Add Diagnosis button on this page and search for either a description of the diagnosis or an ICD 10 / DSM V diagnosis code in the field that appears.

You can add multiple diagnoses by clicking the + Add Diagnosis button multiple times.
Once the assessment is completed and signed it is locked from editing, though an addendum can be added for diagnosis edits and additional notes.