How do I import a wmv video from my PC to my Iphone 5S via Itunes?

iTunes and the iPhone do not support WMV files. You will first have to convert the video to an MPEG4 or H.264 format video.

Handbrake is free and should be able to do this, see https://handbrake.fr

WMV is a widely-used video format on Windows; But as the typical Windows Media Video format, it is not recognized by iTunes by default on Mac. So you may always fail whenever trying to copy WMV to iTunes for Mac. To solve your problem in playing WMV in iTunes, you need to convert WMV to iTunes compatible format .mp4, .m4v, .mov.

Handbrake is a nice choice, or you can search a professional WMV to iTunes converter.