First, unzip the file by double clicking on it. You'll get a new folder titled Wooden Slick Drives. Open that.
To set it as the icon for your HD, highlight the icon you want in the ICNs folder, click File > Get Info (or Cmd + I), then in the new window that opened, highlight the *small* icon in the top left, then File > Copy (Cmd + C).
Next, highlight the folder/icon you want changed, Get Info, highlight the small icon, then File > Paste (Cmd + V)