As far as I know, the Microsoft Wireless Display Adapter (assuming that's what you're referring to) does not work with Windows 7.
If you have some other Miracast device and if your computer supports Miracast and you have updated all the drivers, you might be able to use a wireless display adapter on a Windows 7 computer.
See, for example, the "proposed" answer from "Stefano T" here: http://www.microsoft.com/hardware/en-us/p/wireless-display-adapter along with the following comment from "KimCrct"