Hello DHS, and Welcome to Tom's Hardware!
This is a brand new HDD? The reason it doesn't show up in Windows explorer is that it need to have a drive letter assigned to it first.
Go to Disk Management, and in the lower graphical section, that drive should show up as Disk1 or Disk2, uninitialized, with a black band above the volume itself. Click Initialize, which adds a disk signature to the partition table. Then you can add a Partition, or several Partitions to this drive, and format each with NTFS, default allocation unit size.
Finally you should be able to add a drive letter to each Partition, which will make it visible in W explorer, and ready to store data.