While I am not positive about that particular event, dinner at the HM is one of the many private event experiences you can have at WDW. It is not something say a family a 4 could just sign up for. It is akin to a wedding reception, corporate event etc. Basically all you need is the money, which is considerable, and some guests.
EDIT: I did a bit more research and there is a special event that is occasionaly offered exclusively to DDE members that will allow you to dine in the HM. That article is no doubt a description of the 2002 event. The last time it was offered that I found record of was 2003. The price was $125 per person limited to 70 guests and was sold out in less than 10 minutes on the day it became available.