Note: Running in Zwift is currently in beta, and they really mean "beta". That's why the feature is only available as an "Easter egg" or extra piece of functionality developers cleverly hide for the truly devoted to find. If you do try running in Zwift, be prepared for buggy behavior, lack of support, etc., and most importantly, a totally new running experience!
The gear you need for running in Zwift is pretty straightforward:
1. A bluetooth foot pod
We'll start with the bluetooth foot pod, as this is the one piece that might send you on a detour to the store before running on Zwift. My first impression from beta reviewers was that Zwift only worked with a couple of foot pods; however, as more runners experiment with more bluetooth devices, it appears many different foot pods will work. And as with everything running and triathlon related, you have choices ranging from $ to $$$ to do the trick.
MilestonePod. Somewhat suprisingly, this little square seems to have much of the functionality of its mid-range counterparts. Judging by the success of people using it on the Zwift Facebook groups, I decided to start with this option and see how it goes. Mine is on order as we speak! The one "catch" with the MilestonePod is that you need to contact support for a beta firmware update first, as explained in this MilestonePod video "How to: Pair MilestonePod with Zwift iOS app".
Foot pods & foot pod "converters"—$$
DC Rainmaker covers a couple of options in the mid-range foot pod category:
As far as converters go, if you've already got a foot pod and don't want another, this may be what you're looking for. (That said, I have a Garmin ANT+ footpod already & decided I might as well try MilestonePod as it's cheaper than either converter would be.) DC Rainmaker also reviewed the two most popular ANT+ to Bluetooth conversion options here:
Stryd. Stryd is in a whole different category than the other foot pods mentioned here, as it serves as a virtual power meter for running. Stryd is a Team Endurance Nation partner, so I've been hearing a lot of (good!) things about it, and team members get a sweet discount. If you're a triathlete who already likes using power on the bike, this option would be a worthy consideration.
2. A Treadmill
One of the exciting things about Zwift running is that it doesn't require a special treadmill. The footpod captures the metrics needed "in game", so you're good to go once you've got a comptible foot pod (or converter).
3. The Zwift iOS app
I already had Zwift installed on my computer for cycling, but it sounds like the running function currently exists only on the new iOS app. The Zwift iOS app was easy to install on my iPhone 6+, not so much on my ancient iPad. (First it claimed it needed a newer iOS; after updating, it said it wasn't compatible with the device :-/.) Assuming you have a reasonably new iOS gadget, you should be all set.
The real issue I ran into was figuring out how to find the Easter egg in the first place! A few videos describe clicking on a specific area of the screen until the running options appears, which simply does not (currently) work. The video that finally helped me to get in was this one: "How to run on Zwift!! - Zwift iOS Running App". After that, it should simply be a matter of pairing, then off to the races!