The Apple iPad requires 10 watts of power to charge. As a result, it will refuse to charge from many USB ports on computers, even some MacBook Pro models. If you cannot charge your iPad from the computer then this is the problem. To charge the iPad, you should use the included wall charger.
If you lose the wall charger for the iPad, you will need to purchase a replacement that can supply at least 10 watts of power, so that the iPad can charge. This is an unfortunate design, because it means that the iPad cannot sync and charge with most computers at the same time, but needs to be synced through the computer and charged through the wall outlet. It is especially unfortunate, since it could conceivably sync via WiFi but this is not supported, either.