After installation a new Flip4Mac pane will appear in System Preferences. Make sure you set the boxes to ensure that it will play WMV files in Quicktime.
Likewise there's http://www.easywma.com/wmv/ - "EasyWMV can be used to convert or resize wmv, asf, avi, mp4, mpg, mpeg, mov, m4v, flv, nsv, swf, real media, and vob video files so that they can be used with QuickTime Player, Apple TV, iPod and iPhone." - payware
The only way I can think of attempting a free conversion would be to play a WMV back in VLC and capture it with VLC's streaming mode capture into a different format such as mp4. I have never tried that so I don't know if it will work.
The software that may help you a lot on video conversions is Video to DVD Converter for Mac, which can easily convert wmv files to DVD on Mac without losing quality. And you said that you want a easy to use software, then I think you may need this step by step guide.
I would say this is the best solution i found ever, and this tool is easy to handle even without learning that. Anyway, i found a tutorial aims at solving the problem of burning wmv to dvd just in case. http://www.youtube.com/watch?v=zOW-98GL6NU