Unable to clip movies

Am running Yosemite 10.10.3 on my iMac. I want to trim some movie clips. I have selected the clip I want to clip in the timeline, but when I go to select 'clip trimmer' from the drop down menu under Window, it is greyed out. I've also tried moving the clip end towards the middle to clip it and that doesn't work either.

The movie clips are from my GoPro and I moved them over from iPhoto.

Have been having problems with my movie clips ever since installing the new upgrade from iPhoto to Photo!

Any ideas?

Thanks.