3. Choose the drive letter (Z drive or any open drive, right?)
4. Type in the location like this \\IPAddress\name of shared/external drive (where do you find the "IP address" and the "name" of the shared/external drive?)
The default IP address of your AirPort Extreme base station is: 10.0.1.1. You can also substitute the AirPort's Base Station Name for the IP address. The "name" of the shared drive is what you named it in the AirPort Utility.
In the image below, the base station name is "AEBSn" and the shared drive's name is: Patriot. So, in this case, you would enter: \\AEBSn\Patriot
Additionally, can you access that shared drive from an iPad? If so how do you do that? Is there an App needed?
Yes, and yes. One such app is FileBrowser.