You can delete movies from your camera in three different ways.
1) Reformat the microSD card. This will erase everything on the card and make it ready to record again.
2) After importing videos in to Storyteller you will be asked if you wish to keep or delete the originals. This is referring to the videos on the microSD card. Select Delete to permanently remove the videos from the microSD card.
3) While the camera is attached to the computer, you can navigate to the DCIM folder of the microSD card in your camera and move all of it's contents to the trash. If you are using a Mac, you will also need to take the additional step of emptying the Trash before ejecting the camera.