You need to install an extension for it if you're trying it on your own wiki. If you are talking about a wikimedia project, it's already installed. On your preferences, you should try "Toujours produire une image PNG" (at least until it works). You may be gettting the Tex because your browser doesn't display the image ( \sqrt{2} is the alternative content for the image). Your browser should be configured to display images, even from external sites (if not, you won't be able to see any image, so you would have noticed :P ). Check you can see http://upload.wikimedia.org/math/e/f/5/ef5590434a387b3c4427e09d5b08baaf.png Sometimes filters block some images.
And i do see it http://fr.wikipedia.org/wiki/Utilisateur:Platonides :-)