Why can't i speed up my clip?

Basically i have imported a clip recorded from my XBOX 360 using a Happauge HDPVR 2. I've used Quicktime Real player to trim my clip and in a .MP4 format. I then imported it into iMovie, but now i can't speed up my clip? I get the option of speeding/slowing the clip, but the speeds are greyed out and cannot select them. I dont know what to do? The clip is around 60 seconds long.

Thanks