When you first plug in the board you wont probably wont have a driver for the com port install and windows cant find it at least that's was my experience on win7 64 bit running version 1.8.7 of the IDE
To get the driver downloaded and installed , start up the IDE and go to "tools > board:" and click on "Boards Manager..." at the top of the list. In Boards Manager look for Arduino megaAVR boards and click on the install button.
This will configure the IDE and will download and install the windows mEDBG comport driver ( look for a windows pop up asking permission to install.
Plug in your Arduino wifi rev2 and you should it appear under device manager under ports as a mEDBG Virtual COM port (COM*)
btw mouser.com has the boards in stock I just got mine today.