[Brian]

Hmmm... well, if the machine doesn't have built in wireless, you're basically going to want to make sure there isn't any additional software running on the machine which might be blocking network access (Little Snitch is one example of this sort of software).

Once those possibilities have been exhausted, you'd want to take a look at whatever routers, wireless access points, and so forth between the Mac and the iPhone. If one of them is running a firewall of some sort, that could prevent the two versions of OmniFocus from talking to each other...