Do you mean the taskbar that appears at the bottom of the screen when you double-click the home button :

Or the iPad's dock which contains up to 6 apps and stays the same when you move to different homescreens ?
If the first then the app needs to have been recently used and it will appear there - so having used an app should get it to appear there (if an app is currently open on-screen then when you double-click the home button it won't show on the bar, you will need to back to the iPad's home screen or switch into a different app for it to appear).
If the second then press and hold any of your apps on your iPad's homescreen and after a second or two they should start to shake, and you can then drag (not too quickly and one app at a time) up to 6 apps so that they appear on there - once you have 6 on there you will need to remove one so to replace it with another one.