To log medications, navigate to the medications tracker screen. From there, select the client that you wish to track medication for in the dropdown on the right of the screen. With the client selected, you will see their list of medications (both unscheduled and scheduled) display. Within the Status column, you'll see the opportunity to log whether the medication has been administered or not and why. 
When a status is selected, a pop-up will appear that will allow you to notate more details about the status of the medication. When you've completed this, select "Save" and that medication will be logged and documented with the status you've chosen previously.
The medications that are displayed for each client are entered within the Client Information screen under the "Medications" tab. For assistance with adding medications for a client, please review this article: How to Add Medications to a Client