On OSX you can use the following UserPameters in the config file to get a little more possibilities where the agent can’t help you yet..
Get CPU load in % (as opposed to avg1, avg5 and avg15) – formula takes top output idle % and subtracts it fom 100
UserParameter=cpu.percent,echo 100-`top -l 1 | awk '/CPU usage/ { print $7 }' | sed 's/.$//'`|bc
Check if a service is present if the process list – in the trigger you need to specify the Service name (eg: service[Activity Monitor])
UserParameter=service[*],pid=`pgrep "$1"`; if [ $pid -gt "0" ]; then echo "$pid"; else echo "0"; fi
These are tested on a 10.8.5 system with zabbix 2.2.2