I have downloaded the app for my ipad 3 but cant seem to get the video part of it to work help please

Have you checked the settings?

EDIT:
As I've seen with my research you need to install another application in your iPad to use it as a webcam in Yahoo Messenger, an example is WebCamera.

EDIT 2:
On another side, I've also seen that there should be a button for video call and there should be no problem at all.