Using the Wizards in SonicWall is the key. I would provide access to OWA over SSL, port 443 which is more secure than port 80, HTTP. This will require you to have an SSL certificate installed in IIS for the Exchange website.
Here are the steps to configure port forwarding.:
http://www.fuzeqna.com/sonicwallkb/consumer/kbdetail.asp?kbid=4535