It depends on the type of output you have. Check if you have an open-collector one that can sink 10..20mA. If that is the case, then just hook up an optoisolated solid-state relay. Ie solething like:
http://www.davis.com/catalog/product_view.asp?sku=6847435&pfx=Hook the positive control terminal to +5V and the negative to the open collector one. I assume that you get a relay that can be controlled by 5V.
OTOH. If you are the least bit unsure to work with mains potentials, then ask someone to help you...