You use isin.
Code:
on *:text:*:#: {
  ; This will watch for the word in all channel messages.
  if (word isin $1-) { echo -a $nick used the word }
}
on *:text:*:?: {
  ; This will watch for the word in all private messages.
  if (word isin $1-) { echo -a $nick used the word: $1- }
}

This goes in to your remote. ALT + R in mirc.