How to provide symmetrical power supply with laboratory power supply?
Leave Green disconnected.
Pick a red as your positive. 
Connect its black to the other red. 
Use the other black as your negative. 
The red/black junction is zero. 
Enable the tracking function so your controls affect both outputs at once.


Per request for additional information about tracking function, when you've got a dual supply, and the tracking function is active, it makes the controls affect both outputs "symmetrically", so you don't have to dial them in independently. One output is designated as the master, and the other output is slaved-to (i.e. tracks) the master output.