DX(G) How to debrick a demo DXG?

After reading through all the threads, I still do not know to do it. Will appreciate any detailed help here. (Using an iMac)

1. What serial-TTL cable to buy? 5v, 3v, or 1.8v? Would be great for any link to the ebay listing...
2. The script and files need to be used?
3. The detailed, easier to understand procedures...

Thank you very much for the big help...