The "+omnifocus" thing is using an obscure feature of the email standard called extended addresses. You need to include that along with your username before the @ symbol. For example, if your address is
jgalt@gulch.com, you'd need to send email to "jgalt+omnifocus@gulch.com" for OmniFocus's mail rule to pick it up. Unfortunately, not all mail servers support this feature (*cough*exchange*cough*), so your milage may vary.