Here's the short answer: You can't do it. If I remember right, the 5400s used IDE drives, and the 6100s used SCSI drives. You can't use an IDE drive in a SCSI computer without some modifications.
For what it's worth, to open your 5400, remove the back panel and you'll see the hard drive in the lower left corner. Remove the IDE and power cables, pull the latch under the hard drive up and pull the drive out.
As for the 6100, that's a simple case to open.