Theres a slot on the bottom of the console whats it for?

Was originally planed to have a cd console attached like the Sega cd. They first worked with Philips (giving us 3 zelda games and a mario game on the Phillips cdi), than they worked with Sony. Sony broke away and gave birth to the Sony PlayStation.

It is like Taylor Said ^ It was meant for a CD Expansion, or the Stellaview same reason most Nintendo Consoles have the same port except it was only really released in Japan and are extremely expensive. It was also meant for Nintendo Test Stations, since the disc drive nor Stellaview were released in the U.S. it's primary function became for the Nintendo Diagnostic/Test Stations.

They originally were working with Sony to produce a console that would utilize both cartridge and disc games, however when Nintendo decided to work with Phillips, for reasons that I will not go into detail for, they breached Sony's contract. Sony then later released the PlayStation with the slogan "PlayStation, It Only Does Everything!" it is also the same reason why some PlayStation commercials began with the commercials for Sega and Nintendo and led into theirs, that way they were able to show the consumer that they are better than Nintendo and SEGA.

like the n64, might have been used for a disk drive.

I don't knowI don't know I might go to a trailer and I go to anything I have no idea where it might go to

Like said above, it was a partnership with SONY to make a CD-ROM add-on, but Nintendo saw what happened to the Mega CD (/Sega CD) and got cold feet. This gave birth to the competitor they STILL fight yearly - SONY Playstation!

(So it's a good thing it was never used really!)

There's art of the proposed "SNES CD" and also ONE Prototype found in the wild! I think Ben Heck showed it off? Just look up "Nintendo Playstation Prototype" or similar.

It can be used for hooking up to a computer for modification you have to put a cartridge in then click on the folder "cartridge/empty" (has to be an empty cartridge) and the adapter for the computer is only 18 dollars!

I do not have a Nintendo 64DD I wish I had one.

It was used with the Super Nintendo Excercise Bike and Mountain Bike game. Look it up on youtube. Someone found it at a resale shop. He plugs it into the bottom of the snes which has a wire leading the the bike so you can peddle. First time I've seen that port be used for anything.