I saw that this was added in a previous version, but it looks like it's gone again.

Which version of Ender IO and NEI are you using? What kind of integration do you expect/is not working for you?

2.3.0.421 Ender IO
1.0.5.120 NEI

The recipe button in the Alloy Smelter does nothing when I click it, I can't see ANY of the recipes.

I just checked (Ender IO 2.3.0.423 and NEI 1.0.5.111), it works for me. So it is not generally broken from our side. Could you please check your logfile for any errors or warning messages? Both at startup and when you click the recipes button. And: Does the recipes button work for the other Ender IO machines? Can you find the alloy smelter's recipes in NEI when looking at the recipes for things (R key) or when looking at the uses of things (U key)?

If you cannot find anything in the logfile, please: Create a new Creative world and place an alloy smelter in it, re-start the game and enter that world, click the recipes button, exit the alloy smelter GUI, crash the game by holding F3+C for 6 seconds, and send us the logfile, e.g. by attaching it to this ticket.

It happens both with the smelter recipe button and R /U in NEI. Neither shows ANY alloy recipes. I'll get that new log after work tonight (8h).

Looks like SAG Mill Recipes aren't showing up either, probably all Ender IO machine recipes.
Is 423 released yet? Or is that a beta/dev build?

Confirmed cannot see recipes in the Alloy Smelter, SAG Mill, Vat, or anything else. Will try downgrading NEI as that may have been the cause....