As its an AC power (alternating current) its doesn’t matter the polarity, so just connect it as shown in the picture.
One suggestion, replace the power cable with a 3 wire one with proper ground (PC power cable will be perfect). The ground connector on the power supply is just next the N on the left regarding the pic (Lines in a Circle diagram), the ground cable must run directly from the wall socket to power supply without terminated by any switch. Also power supply’s case(the ground will be terminated to that) and your alu frame should have a direct contact for proper earth.
Second suggestion, if you have an old faulty PC power supply you can use its power connector to attach it to the ALU frame (also its switch can be used).
Just make sure that all wires are properly sealed.
Here is an idea: