Or if you don't want to copy-paste manually every time,
drag this link to your browser's bookmarks toolbar: ☹ I Don't Like
Now go to Facebook, click on a comment field, click "☹ I Don't Like" and enter.
See how to do it here.
Note: idontlike.info is not a real dislike functionality. It's just a picture of dislike button that you can post on Facebook.
idontlike.info is in no way affiliated, sponsored or in any way the property of or responsibility of facebook.com.