On the barracuda you go to the Basic Outbound page. You add the IP of your Exchange server to the "Relay Using Trusted IP/Range"
IP = 192.168.0.3 netmask = 255.255.255.255
On your Exchange server you set your smarthost so that is sends outbound mail to the Barracuda
Move the radio button to "forward all mail to this connector" and enter the IP of the Barracuda as the destination.
NOTE that exchange is temperamental and often will not actually use the smarthost when you save it. Often you have to reboot the exchange server to get it to start working.
You can tell it is working by going to your Barracuda message log and do a search for source IP contains 192.168.0.3, This will show you mail coming to the barracuda from your exchange server.
NOTE it is important that your router be configured to allow outbound port 25 traffic (SMTP) from the Barracuda IP. You MAY need to create a rule for that.
I can't make it any clearer than that.