You will need either a serial or USB service cable. This is wired differently to the one that comes with some SE's, and can be bought for a few quid from ebay. The software is free on the net now. It's called "usb smart"
The DCU60 usb cable that comes shipped with the k750 / d750 will flash the phone, and can also be used for de-customisation without the need to rewire it.
The phone will not change to developer mode (brown) with this configuration, or without the 3v needed for the flash I.C
The phone is first changed to developer mode (brown), then the software unlocks it for you. Finally it converts back to RED (normal).
Try it. It cannot do any harm to the phone (unlocking). Then have a look here for a cheap cable that will work www.fonefunshop.co.uk