Your users are under Settings then Users. Users who are trainers will have a "Trainer" indicated under their names. Employees who you have indicated are not trainers will only be listed as "Employee" You can always change the status of your users by clicking on Actions > Edit User and check or uncheck "This user is a trainer" option.

You can access the trainer availability setting tool by clicking the on Actions > Manage trainer availability under each trainer's profile.

On the next screen you will be shown a calendar for the week. You can switch between the weeks you are viewing by clicking on the arrows next to the dates. From here, you click on the time and day you wish to set an availability timeslot for and you will be asked if you wish to set a recurring availability (an availability for the same time every week) or a single-day availability (only for that day): 

After you choose if you wish to set a recurring availability or a single day availability, you will be asked to set the timeslot, time between bookings (a buffer time between two appointments in that timeslot), and a location (a location must be selected.)
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You will also have the option to add another time slot to create multiple availabilities within the one day with one simple step: 

This will then show on the calendar like this:

Go to the "Calendar" menu, "View Calendar", then click the "Trainers" tab: 

You will then be taken to the trainer availability tab where you will be able to see the trainers, what times they have allocated that they are available, and which location they are available at: 

You can add an appointment the regular way by clicking the orange "+New" or by clicking the block of time in which you can then set the appointment for the time within the block:

Then when you click "Submit" the appointment will show on your regular calendar and also on your Trainers calendar: