Verify that you installed the hard drive properly in the enclosure. Also make sure that your external power adapter (if required) is plugged in as well. Double check your USB (I assume cable is connected on both ends and try another port (and even another computer) to see it the drive works at all.
BTW, can you tell (just feel the enclosure) whether the hard drive is running or not? If not and all seems to be connected on the outside of the enclosure, you may need to open the enclosure to verify that both your data and power connectors are properly connected.
Good luck!