To log in as one of your trainers, navigate to the Trainers page. (See image below.)
Once you are on the trainers page, click the Login button (under the column titled "Login As This Trainer") on the trainer you wish to log in as. (See image below.)
Once you click login, you will be able to see all of this trainers clients, workouts, meal plans, and everything that they have created.
To go back to your gym/studio management account, click the Change Back To Gym View button in the top right of your screen. (See image below.)