I am unable to activate SMS Relay (aka Text Message Forwarding) between my iPhone 5s running iOS 8.1 and (a) my iMac running Yosemite and (b) my iPad Air running iOS 8.1. Neither the iMac nor the iPad displays the activation code which is supposed to appear when Text Message Forwarding is activated on the iPhone.
The iMac is shown as "iMac (24-inch, Early 2009)" on the About This Mac summary, so is no doubt too early to support SMS Relay (elsewhere I've seen references to 2011 models being the earliest supported). So my first question is:  Am I out of luck even trying for SMS Relay on my iMac?
My iPad Air is only a few months old, which I would expect to meet the requirements for SMS relay. However, it is a Wi-Fi only model, so my second question is:  Is it necessary to have a cellular iPad for SMS relay to work?