If you are using a single amplifier, you need to run a jumper cable between each of the red (hot +) and each of the black (ground - ) binding posts.
From amplifier + terminal -> red binding post on speaker-> other red binding post on speaker
From amplifier - terminal -> black binding post on speaker -> other black post
From amplifier + terminal -> red binding post on speaker-> other red binding post on speaker
From amplifier - terminal -> black binding post on speaker -> other black post