IIRC, the Mac Plus does not supply Termination Power (which is different from Termination). Check the hard drive for a jumper to enable Termination Power (Term. Pwr., perhaps) from the hard drive.
Otherwise, do you know that you have a good drive and good SCSI cables?
Oh, and if the drive isn't initialized, you'll need to run a disk utility first to prepare the drive. That means booting from a floppy first, unless you have another external hard drive. If the drive is not from Apple you'll need a third party disk utility such as FWB's Hard Disk ToolKit or Silverlining, or APS's Powertools.