Get a warning messages on system logs ? I started for some reason OSX's Log console and just found out someone was attacking me with ssh connexion's 

I had over 500 of these messages (just the user name was chaging) in 5 minutes : 

23/09/13 14:55:04,638 sshd[18830]: error: PAM: unknown user for illegal user crystal from 193.183.98.218 via 192.168.0.12


Do you know a way to get some kind of warning popup when a process sends a lot of logs like here ?
I do not know how to monitor the console for a specific message and send an alert.

However, there is a alternative approaches to monitoring or even block a specific ssh activity.

Without taking any credit (other than finding it), I would advice you to look up at some suggested solutions here that might apply to your situation.

Getting notified when someone logs into a server using SSH or Remote Desktop