Packaged binary versions of Wifi are available from http://github.com/diva/d2/downloads in two packages: as part of the standalone Diva distribution (D2) of OpenSimulator, and another package for use with a ROBUST server grid implementation of OpenSimulator. The latter package (Wifi for Robust) is NOT intended to work with standalone implementations of OpenSim.
Obtain the latest Wifi module distribution zip from http://github.com/diva/d2/downloads and place the files as instructed in the equivalent locations in your OpenSimulator directory.