I would like to put a password on the hard drive so that others sharing the wireless network cannot access said drive. How do I do that?
This can be accomplished by using AirPort Utility. By default, the password used to access an attached hard drive is the same password as the AirPort Extreme device.
You can change that to a Disk Password, which would mean that users who wanted to acccess the hard drive would need to know the Disk Password.
Assuming that you are running Lion or Mountain Lion on the MacBook.....
Open Macintosh HD > Applications > Utilities > AirPort Utility
Click on the AirPort Extreme icon, then click Edit
Click the Disks tab
Click on the Secure Shared Disks selection box and change that from "With device password" to "With a disk password"
Click Update to save the changes