Local or remote?
or this:
http://forum.mikrotik.com/t/run-scrip-when-traffic-is-detected/138712/1
or this:
http://forum.mikrotik.com/t/trigger-event-on-port-access-e-g-send-wol-packet/51866/1
Or using the /tool wol:
http://forum.mikrotik.com/t/wake-on-lan/57248/1
Just in case, there are some devices that won’t “respond” properly to a broadcast WOL, so there is a manual workaround to send unicast WoL packet:
http://forum.mikrotik.com/t/tool-wol-target-ip-address/175220/1