An AirPort Extreme is a router only.......it has no built in hard drive. If the product that you are calling an AirPort Extreme has a built in hard drive, you have a Time Capsule.
If you are trying to connect a cable from the USB port on the Time Capsule to your Mac, this is not going to work.....since the USB port on the Time Capsule is there only to add another USB drive for additional data storage.
In other words, you can't connect a USB cable from your Mac to the USB port on the Time Capsule and see what is on the hard drive of the Time Capsule.
If you want to try to get at the data on the Time Capsule hard drive, you are going to need to connect the Time Capsule to your existing router using a wired Ethernet cable connection, then reset and reconfigure the Time Capsule to function on the network.
Even if you do this, you may be able to see the backup file on the Time Capsule, but you likely won't be able to see what is inside the file unless you will be using the same Mac that made the backups on the Time Capsule.
We can tell you how to try to do all of this, but we can't offer any assurance that you will be able to do what you ask.