Simply put: Forget it.
The kit lens starts at 18mm which is a decent focal length for wide angle. If you need it wider then get a real Ultra Wide Angle lens. There are also lens rental shops (see Marketplace) so you don't need to buy.
Since light passes through the lens it somehow works. But the question is: how well does it work?
See for yourself: http://www.clubsnap.com/forums/threads/477934-Using-Wide-Angle-Tele-Converter-on-your-camera
(Hint: it's a sticky thread, right on top of the Newbies Corner.)
The distortions introduced by these converters are worse than even the cheapest DSLR kit lens ever did. Why would you like to have such an image spoiler mounted on equipment where you spent so much money on? Unless you need the effects for certain purposes.. but then there is Photoshop (and other tools) to get these effects in a well controlled way.
I had tried using a 0.5x macro with wide angle conversion lens fitted to a Canon EF-S 18-55mm f/3.5-5.6 IS lens and during autofocusing, you can clearly feel the DC micro motor straining under the weight of the set-up. I sincerely believe IMHO fitting one to a Canon EF 50mm f/1.8 II Lens will stress its autofocusing DC micro motor even further.
Not to also mention the horrible degradation of image quality (additional induced perspective distortion, loss of sharpness, sever chromatic aberration etc. etc.) associated with the use of such wide angle conversion lens.
There are many instance that I seen shops (even reputable ones) are pushing such converters to new camera buyer. In my heart, I really wish to warn those victims to be not to buy. But I can't do it in the stall. Can only stood there watch their money fly away into the cashier.....
dun be another fool to buy those convertors. if they are good, then true blue wide angle lens will be out of business. u pay what u get for. those convertors only works back with those point and shoot cameras, but dun works well on DSLR lenses.