Monitor Console.app for a string and notify when found
I would not use Console.app.  Instead use Terminal.app with the following command:

tail -f /var/log/system.log | grep "string you are looking for"


then leave that window open and you will see all matching entries in that window as you work.

Also know that when the system.log file is rotated (due to size/time) tail will stop.  you will need to restart the command.