You can install widget into the Notification Center by opening notifications of your iPad by sliding your finger down, careful to start at the very top of the screen, and then tapping the 'Edit' button located at the end of your active notifications.

The edit screen is split into those widgets that will display in the Notification Center and those that are installed on the device but not currently showing with the other notifications.
To install a widget, tap the green button with the plus sign next to it. To remove a widget, simply tap the red button with the minus sign and then tap the remove button that appears to the right of the widget.
Once the widget is installed, it will be displayed when you open the Notification Center.

Will There Be a Separate 'Widget' Store?

Apple has implemented widgets by allowing an app to display a custom interface within another app. This means a widget is just an app that allows part of itself to be shown in another app, which in this case is the notification center.
If you want to see sports scores in your notification center, you can simply download a sports app like ScoreCenter from the app store. The app will need to support becoming a widget in the notification center, but you don't need to install a special version of the app. Once installed, you can configure which apps you want to show in the notification center through the iPad's notification settings.

See more at: http://trendymediablog.com/2014/09/how-do-i-install-a-widget-on-my-ipad/