Click on the Time Capsule icon under the SHARED heading, and if your drive is recognized, it will appear as a folder in the window to the right, along with a folder named "Data" representing the Time Capsule drive....assuming that you have not renamed "Data".
If the USB drive does not appear, then you need to check the formatting of the drive. It needs to be HFS+, also known as Mac OS Extended (Journaled) or FAT32.
A Windows format of NTFS or exFAT is not compatible with the USB port on the Time Capsule.
If you have confirmed that the drive is formatted in HFS+ or FAT32 and it still does not appear, you may need to use a powered USB hub with the drive.....even though the drive might have its own power supply.