How to connect b52 ls 100 to its matching cabinet?

people who know guitar half stacks, I have a b52 ls100 head and ls400 cabinet. On the back of the cabinet, there is a mono/stereo switch and I dont know how to connect the head to the cabnet. i have one speaker cable and i want to know if i can switch the switch to stereo with only one cable(there are two cable holes in the cab) and i dont want to screw up the amp. Also there is a statement saying do not switch to mono with stereo terminals in use. could you explain that. Im guessing i need two cables to use the stereo but i may be wrong. People with this half stack, PLEASE ANSWER. thanks