We have no idea what you have tried, but if the drive is not formatted correctly, it will never be seen.
Start by verifying that the drive has been formatted correctly in either Mac OS Extended (Journaled) or FAT32.
The drive will not work with an AirPort Extreme if it is formatted in Windows NTFS or exFAT formats.
Check the documentation that came with your drive to learn about how it is formatted at the present time.