I know how to DD but alot of the time I feel like it's not very threatening . And also when im playing a fox they just do a running shine in the tip of my dash dance. any tips on what i might be doing wrong?
Here is another view point from my understanding. If dash dancing is nothing more than moving the character left to right at different lengths, then why should anyone be worried at all that you are moving left and right alternatively. I'll just walk up to you and attack. Now, if all you are doing to counter this is to keep running away, I'll just keep walking up to you until you run yourself off the stage or you cannot run away anymore, then attack. In the case of your example, fox just runs faster than you can run away.
The movement without other actions is pretty meaningless. Although, some people who do not recognize that will still lose to a constant "I'll wait until they do something, then move out of the way strategy from marth". Here is a quick example:
http://www.youtube.com/watch?v=kREm4Bsw58M @1:11 - Marth first pushes forward and spaces with Dtilt after fsmash. From failing to connect Marth starts to DD and waits. But, recognize that Marth has the running room of about half the stage and is moving back and forth with long strides. On each pass forward Marth could potentially Dtilt or grab sheik at this moment from the dash. Thus, you see Sheik's spot dodge a couple of times. However, realize as well that Marth on each pass forward could potentially double back with short dash to retreat from Sheik's dash grab or dash attack should she choose to attack. This sort of illustrates the some of the benefits of a dash dance. You can switch between getting a hit in and waiting for an attack to come at you.
However, it really only works if you combine both elements I believe. If all you are doing is constantly retreating an opponent has no reason to fear any sort of retaliation from you and can advance forward unimpeded until you completely lose all stage to run away back from and end up on the ledge. At the same time if you are committing to an action too much such as grab you can get caught in your opponents ability to simply wait for you to mess up. Eventually, your opponent might realize that if they are not careful they could mess up and get ruined by a dash dance. At this point, a dash dance by itself becomes a bit threatening. From there a dash forward in itself becomes aggressive and can act as a push on your opponent. As in you are attacking your opponent with movement, but without fully committing to actually using any sort of move at all (shield, attack, jump, grab, etc.)