You use standard ice cream mix, whatever you'd usually use in your ice cream freezer. It's in the metal bowl, even though you can't see it because of all the fog. :D There's more in the plastic container with the blue lid labelled "Dark Chocolate" -- it's best in smallish batches. The metal bowl is inside another metal bowl, and the whole pile is on a table which is covered with a towel because the hotel owns the table and you never know. :P The metal pan (the one covered with frost) contains the liquid nitrogen. You pour it slowly into the bowl of ice cream stuff while someone else does the thing with the mixer. In previous years he's used a spoon (metal) but I guess the electric mixer is easier. The liquid nitrogen freezes the ice cream stuff (actually, it turns out like soft-serve, which is as frozen as it can get in a vessel exposed to the air) then boils off, so there's no extra nitrogen actually left in the ice cream when you're done and it tastes perfectly normal.
The lady holding the pan told us about a convention she ran a few years back where they made liquid nitrogen ice cream in the con suite. She was outside and downstairs having a cigarette when she heard this BOOM! (the window was open so it was quite clear) and then this chorused, "Ooooooo!" [facepalm] Whenever a group of fans is that impressed with something, you know there's most likely something wrong. Turns out they'd tried to make the ice cream in a pressure cooker and (surprise!) the liquid nitrogen froze the pressure valve shut. It exploded, sending shrapnel and ice cream goop flying all over the place. She didn't say but I imagine that added a nice amount to the check they had to write the hotel at the end of the con. Luckily no one was hit by the shrapnel (pretty miraculous, actually) but this one fan who'd been watching ended up covered in ice cream goop. Luckily (again) he thought it was really cool and didn't sue anyone. Whew! LOL!
Luckily, the guy holding the mixer, who's in charge of the concocting of liquid nitrogen ice cream at the BayCon FOB party each year, is much smarter than the people who tried to use the pressure cooker and we've never had an accident, yay. :) The ice cream turned out pretty good, too. :D