Surface RT does not support Miracast. Surface Pro, and all "2" variants do though.
They said MiraCast is supported in W 8.1 but I've yet to get it to work on a Dell XPS 12, Surface RT or Surface Pro with (3 different) Netgear PTV 3000's using the latest firmware. I looked at how people said to connect to it but I never got past the pin prompt, horribly frustrating. WIDI or MiraCast + windows 7/8/8.1 != AirPlay ease of use.
Can you email me more details? (see blog) You can use DxDiag - Save All Information to see if your system is Miracast ready.