Make sure that the drive has been formatted correctly to work with the Time Capsule.
That means a format of Mac OS Extended (Journaled)....highly recommended.....or FAT32.
Once this is checked, open Macintosh HD > Applications > Utilities > AirPort Utility
Click on the Time Capsule icon, then click Edit
Click the Disks tab at the top of the screen
If the USB port on the Time Capsule can supply enough power to the hard drive, you will see the name of the drive displayed in the window, per the example below:
If you have the correct format on the drive, and it does not appear, this is an indication that the USB port on the Time Capsule (which is under powered), does not have enough power to work with your hard drive.......even if the hard drive has its own power supply.
In this case, you will need to use a powered USB hub with the Time Capsule to provide enough current for the hard drive.
Now you should be able to access the drive using the Finder to mount it on the desktop.