Have you partitioned and formatted the drive? If not, then it won't show up in Explorer with a drive letter. You can partition and format the drive using Disk Management as follows:
- Start -> Right-click "My Computer", select "Manage"
- Click "Disk Management" in the left pane
- Find your new drive in the LOWER portion of the right pane and right-click on it to create a new volume (which partitions it) and format it.