Probably the easiest route to take, if you aren't of the "my data is too confidential to store on someone else's computer" persuasion, is to sign up for Omni's sync service, which you can do by going to
http://sync.omnigroup.com and requesting an invitation. As long as your iPad and Mac can both access the internet, you shouldn't have any of the trouble that can sometimes arise trying to use the WiFi sync.
First thing to check if trying to get WiFi-based syncing working is that you've get OmniFocus on the Mac running, and have designated it as the server by clicking the "Keep the sync database on this Mac" button in the Sync preferences in OmniFocus. Then, you need to make sure that the Mac and the iPad are connected to the same network. Check the Wi-Fi section of the Settings app on the iPad and the Network Name in the AirPort section of the Network preferences in the Mac's System Preferences and be sure that they match. If everything mentioned here is okay, but the iPad still can't find the Mac, contact the Omni support ninjas for help at 1-800-315-OMNI or
omnifocus@omnigroup.com and they can walk you through the troubleshooting.