So how does this thing work? How do I know what materials I need to order?

If I wanted to make a ceramic gun for instance, do I just dump some ceramic material, upload the schematics in and the forge will shape it to the components I need?

Uh, no.
The Forge will spit out the PARTS for the gun.
Not a complete Gun.
You will need to assemble it yourself

You order the goop (feedstock) for the forge. and feed it the schematics and out come gun parts.

"Th e costs for feedstock are about the same
as for the relevant parts that would be used in a modifi cation
without desktop forge support"

Basically, I view it like a rapid prototyping machine, you install the CAD design to the machine and it will make the parts for you. It can't make the entire thing from one piece, but can more readily make the individual pieces. From which you can assemble together to form the piece. Possibly some structures can't be made by the forge itself in which case you can order it, but for the most part it you can make the majority of the items from the same feedstock (goop as someone mentioned).

It's basically a really advanced three-dimensional printer.

which is a rapid prototyping machine in general.
http://en.wikipedia.org/wiki/Rapid_prototyping
Hmmm... all this talk about the machines is reminding me of work. wheee.

You get to play with fun stuff at work O.o

i wish, no I get to read and review the schematics and the inventor's possible innovation. Then after a review of the databases I get to write basically a report saying if I think it is or not, then wait several months for their lawyer's response. I stay in front of a desk all day. *sob*

Ah, okay, i thought you got to play with machines that can produce any toy you can input into their computer. poor you, i feel your pain. I do Techsupport.
But hey, Einstein had the same Job once neh? Invent something yourself, and they will say YOUR name next in this example ^^

So if this is what we can do today...
...Why are SR v4 ones [with 50 years of development] not much better ??

Nano Forges similar to those in Diamond Age are what I'd expect.

at least you aren't sitting here staring at a mainframe all night long.

I see at least two reasons :
1) They ARE more advanced. They can use and assemble a wide range of materials, with a finer resolution, directly integrate functions, embed section of vastly different materials... making them much more flexible than today's models.
2) As exposed in the desription of those devices, the corporations quielty sat ont the technology for years because it did not fit into their model of consumer-producer relationship.