You have to make a cast with a stone tool part. Learned that the hard way. Definitely make an ingot cast straight away. It's kind of a waste of gold tho to use it to make casts, so I'd definitely save up alum/copper for that instead. The only parts you can't use are the fletching and bow string (because they have to be feather/leaf or string/cloth material) to make casts with. Make sure you've got stone tool parts though. Wood doesn't work.
I'm having a lot of trouble making the casts and casting the parts though, but I think it's because the stuff gets buggy if you try to add drains later on. ( Ugh. I wish this could be updated within hexxit. Otherwise I might just have to mod my vanilla to be a pseudo-hexxit.) It's pretty frustrating, so make sure you don't add any drains on the corners or destroy any blocks after you start using the smeltery. Just to be on the safe side.