iMovie 11 does not natively edit 50P material. You can convert the material to 25P using a converter like ClipWrap or Voltaic HD.
However, there is a way around this limitation if you really need good slow motion and you don't mind hacking a plist. See this blog post.
I have not tried this myself.
What I do is set the framerate in the plist file to 50. Then I convert my .mts files to mp4 files using remux (just a rewrap, no recoding). Then I import the mp4 files into Imovie. My macbook does not work well with the h.264 codec in the mp4 file so I convert to the .mov file using the import option in Imovie. Even then my macbook struggles but it works on short films.