What is the range of the Airport Express?
Apple's specifications of 150 feet don't really mean much since this measurement taken in free air with no other wireless networks around, and no obstructions like walls or ceilings in the signal path.
As you might imagine, it is not possible to predict wireless coverage in a home due to all the unknowns.....how many walls, ceilings, etc. must the signal try to pass through? What is the construction of the walls and ceilings? Are there other wireless networks or camera/security systems in the area? Cordless phones?
The bottom line here is that the iPad should pickup the signal about as well as your MacBook Pro, so if you've already tried powering down the Express and then restarting it to reset the device and the wireless signal....and the iPad still won't connect...you might want to take it to the Apple store to let them have a look.