Can't Delete Video Files from my Macbook Pro hard drive
Open Finder and go to your Home folder (named after your User Account).
Find the Movies folder.

You can drag it into Finder's Sidebar for convenience.
In the Movies folder, you can delete anything.
Make sure to quit all programs before deleting something, so that the files are not in use. In other words, don't be listening to iTunes or whatever while you're deleting media files.

In some cases, a file may be in use, even if you've quit the relevant app. If this happens, log out and back in, or reboot the computer, then attempt to delete the files.
Be sure to empty the Trash after you delete the files, otherwise the space will not be regained.