UPDATE x 2: It now appears as though whether or not you have the WWE Network on an older Roku device just depends on the device itself. There are conflicting reports which seem to indicate that the network may be slowly rolling out on all older devices, but hasn't appeared on all of them yet.
A reader by the name of Charlie Gregg wrote: "I read your post regarding reader feedback of the WWE Network not being available on older Roku devices. The Network was not showing on my Roku device either. Last night, I went on the Roku website and used my email and password to log in to my account on their site. I searched for WWE Network and was given the option to add channel to my device. I just checked and the WWE Network is now showing as one of my channels. So, this is probably what those who don't have it showing up on their device will have to do."
However, a Roku representative stated that older devices will not carry the network, as the only devices compatible are Roku LT, Roku 1, Roku 2 and Roku 3. Only Roku software versions 5.3 or higher can get the Network.
However, another reader noted that the WWE Network is available on the Roku XD, so the reports are very sketchy right now just how many devices it is available on and which will ever carry the service. WWE has yet to comment and clear up the confusion.
Your best option would be to just check for yourself and see if it's there.
UPDATED: The WWE Network isn't available to add as a channel for the Roku yet on some older Roku devices, but it is available on the more modern devices. According to WWE's promotional video for the Network, older Rokus would carry it.
ORIGINAL: The WWE Network has reportedly already been added to Roku, PS3 and PS4. So you can now add the channel to your devices. However, you can only start subscribing to the channel on Monday (Feb. 24) to look at content, but you can add it to your devices now.