I am developing an application to choose and and play videos in iPad. I want to choose the videos saved in the default Videos app present in the device. How can I access tho
With iOS development, you can't always do exactly anything you want to do in the phone. Accessing other apps is one of these things. In this case you want to access the data of a built in iOS-app. This app is just like any other app, except it is made by Apple, and it doesn't mean that you can access it's data.
Apps have their own data and apps cannot access data from other applications. (unless it's made available by the app somehow, for example via a web server)
If you are making your app with the intention to release it for official use, what you´re asking can not be done. You will have to make the best of your situation, make your app better than the built-in app so that the users use your app instead of the built in one.