To connect to the projector, first make sure it is on. If you don't know where the remote might be, there is a power button on the projector if the remote is missing.
Once the projector is on, you need to connect your laptop to the Apple TV that is connected to the projector.
To do so:
1) Select the switch-looking icon and then 2) Screen Mirroring
Next, find the desired Apple TV you want to connect to in the list.
You should be prompted to enter a code. This code should be currently displayed by the Apple TV you just selected. When you enter the code properly, you should see your laptop screen mirrored.
To disconnect from the projector, simply repeat the process above. Click into the switch icon, select Screen Mirroring, and then select the Apple TV / projector that you are currently connected to. This will disconnect you.