Is this a new drive? If so, you'll need to partition and format it before Windows will assign a drive letter to it, and it needs a drive letter before it will show up in Windows Explorer.
Click "Start" -> Right-click "Computer" and select "Manage"
Click "Disk Management" in the left pane
Scroll to the BOTTOM of the middle pane and find your drive. Right-click it and create a volume (this partitions the drive) and then format it.
Once you've done this, it should have a drive letter and it should show up in Windows Explorer.