From your 'My Hub' page (please see below)
On the left hand side of your account, you will see the side menu in which you can select 'Workouts'.
Click the workout that you would like to add a superset/giant set to
You can then hit the 3 horizontal grey dots in the top right of any exercise and select 'Group Set'
Which will allow you to pick the exercises you would like to add to a superset/giant set.