Make sure you have the latest Universal Control for the 1818 OS X installed: http://www.presonus.com/products/AudioBox-1818VSL/downloads
Open Logic, and go to Audio I/O Assignments.
Set the Output to "Output 7-8". Outs 7&8 on the AudioBox 1818 VSL will allow you to monitor from the headphone jack.
To also be able to hear audio from outs 1&2 at the same time, please a check mark beside "Mirroring".