You have to upload the workout to your device, and then, when you are going to do it, start it from the menu of the concerned activity. Typically it is in the Activity menu » Training » Workouts » your workout » Do Workout
If you schedule the workout in Garmin Connect, normally the scheduled workout pops up automatically when you start the respective type of activity. Please note that the workout must be of the identical type as the activity. For example you will not see workout of type 'Other' under 'Pilates', or a Run type workout under Hiking, etc.
Once you complete the workout, it will appear in your calendar, and it will be also visible in the Activity details.
