That seems weird - as I believe the Tapatalk app (once its on your device) just offers up the forums the app can read/use. In other words, it doesn't differentiate by device - if Tapatalk has the forum, it's on all devices.
All I can think to ask is - have you run a search for the forum? Sometimes, the wording is not as you would think (e.g. when finding/loading this forum, I couldn't put in iPadForums.net, I had to generically search for "iPad forum").
Sorry, but that's all I've got.
Marilyn