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.
Is this a brand new drive? If so, you may need to partition and format it. Without that the drive will not be assigned a drive letter and it won't show up in Windows Explorer. (CAUTION - this DESTROYS all data on the drive)....
Click "Start" --> right-click "Computer" and select "Manage".
Click "Disk Management" in the left pane.
Scroll down to the BOTTOM of the MIDDLE pane.
You should see your new disk there with a label such as "Disk 1" or "Disk 2", etc. Right-click on the label in order to create a new volume (which partitions the drive) and then again to format it.
Once it's formatted Windows will assign the next available drive letter to it and then it will appear in Windows Explorer.