You can't use unpartitioned space, it needs to be partitioned and formatted before it can be recognised and used.
Go into Disk Management by right clicking on My Computer and picking Manage, then pick Disk Management from the list on the left. In the right hand pane you'll see your HDD as a graphical stripe with the C: drive at the left and the unpartitioned space at the right. Right click in the unpartitioned space and pick "New Partition", make sure the details in the box look OK, give it a volume label if you want and check the Quick Format box, then hit OK. The new partition should format in about 30s and Windows will assign it a drive letter, probably e: if you have a DVD drive on d: - you can swap these by right clicking and choosing "change drive letter".
Once you've done this the new partition will appear as another drive in My Computer and you can put whatever you want onto it.