I don't have a Time Capsule; I have an AirPort Extreme Base Station, which is similar except for not having the built-in storage. Since your question got moved from the initial category (where you posted it) to Time Capsule, maybe someone else who has a Time Capsule will pop by with some advice. If you don't get any more replies (other than from me), you may want to summarized what you learned so far, and post a new question (in this category). New questions often get more views and responses.
With a Time Capsule, you can share the built-in hard drive as general network storage (not just for Time Machine purposes), is that correct? Does that work for you? Can you set that up using the latest version of AirPort Utility?
If you set that up, I'm guessing it would appear in a Finder window in the same way. The Time Capsule appears in the Finder window sidebar, under SHARED. When you select it, the Time Machine's built-in shared volume appears to the right, and you can mount it using whatever security settings you used. If you have an external drive attached to the Time Capsule as well, and it is set up to be shared, it appears as a second shared volume. It is still just that one Time Capsule under SHARED in the Finder window's sidebar, but to the right, there are now two shared volumes. But I don't have a Time Capsule, so I'm not sure about that...
I don't know why the latest version of AirPort Utility does not see the external drive volume? If possible, I would try another USB storage device (even if it's a 4GB thumb drive), to confirm it does work, and it's not a hardware problem on the Time Capsule's USB port.
Also, if you have not tried so far, you may want to reset your Time Capsule (to give it a fresh start).
http://support.apple.com/kb/HT3728
This also has some relevant information about attaching a USB drive.
http://support.apple.com/kb/HT2426