Issue with Quicktime

I've got an issue with Quicktime. When it plays the video, the picture stutters. I panicked, as I thought they were corrupted!! but they play perfectly in other players. So I assume it's Quicktime that's cr*p.

I've downloaded Cisdem Video Player, and that works perfectly. The issue I have though is how do I ensure that it's the default player?

Cisdem is not showing in the Applications folder, So I can't set it. Any ideas?

Thanks

Will