"How do I move songs from an external drive to iTunes?"
There are two possible answers to this question depending on whether you want the songs available in iTunes anytime the external drive is connected to your computer, or if you want the songs available in iTunes even when the external drive is disconnected to your computer.
Adding Music on an External Drive to iTunes
If the external hard drive is always connected to your computer and you simply need to add music from the drive to iTunes, the solution is quite simple. With iTunes open, go to File > Add Folder to Library and choose either the entire hard drive (if appropriate) or the folder where the music files are stored on the drive.
Copying Music from an External Drive to iTunes
If what you want to do is make music on the external drive available even when your computer is not connected to the drive, you'll need to take a few additional steps. The first thing to do is to copy all the music from the external drive to a folder watched by your iTunes software. This is typically your Music or My Music folder. Once you do this, you may have to either close iTunes and re-open it in order for iTunes to find the songs, or you may have to choose File > Add Folder to Library and select the folder you just copied music into before iTunes will recognize the songs.