Now all you have to do is add more brackets so it'll work. Like this

On *:text:!advertizezing:#:{
if ($nick isop $chan) {
set %ad $rand(1,3)
if (%AD == 1) { describe # $nick for all you catering need visit http://straubscatering.webs.com/ }
if (%AD == 2) { describe # straubscatering is a sister company to CMangels inc }
if (%AD == 3) { describe # $nick straubscatering is willing to travel through out the world if price is right}
}
}