How do I add a 3 1/2" Floppy Drive to my AT&T PC-6300 PLUS?

[MR. PORSCHE]

         HELP!  HELP!  HELP!  HELP!  HELP!  HELP!  HELP!  HELP!

       I recently purchased a used AT&T PC6300 PLUS.  The previous
       owner installed a second 20MB Hard Drive in place of the
       1.2MB (5 1/4 inch) Floppy Drive.  He installed it (the old
       Floppy Drive) in an auxiliary housing, which also contains
       its' own power supply.  But the housing's Power Supply is
       not used, since he extended the power from the PC, and has a
       ribbon cable connected from the PC to the 1.2 Floppy Drive,
       with an "edge" connector.  The ribbon cable has a "twist" in
       the middle and works fine in the old configuration.

       Since the new housing can accomodate a second drive, I
       bought a 3 1/2 inch, High Density (1.44MB) Floppy Drive and
       also a new MS-DOS Version 3.3, because I was told that it
       "supports" 3 1/2" drives.  I was also told that a HD 3 1/2
       would support the standard 720K floppies.

       The problem is, I don't know how to connect the new drive
       and make it function as Drive B:.  First I moved the power
       and ribbon edge connector to the new drive.  The LED on the
       front of the new drive remained lit.  The drive accepted a
       small floppy but couldn't format it and gave me an error
       message.  I received no instructions with the new drive, and
       am in dire need of help.  It should be fairly obvious that I
       am a new kid on the block and require simple, straight
       forward directions.  If none is forthcoming, look for an
       upcoming ad selling a "never used" 3 1/2" HD Drive.

       Any advice will be gratefully appreciated!

       Marv