Are you holding any kind of ore stack in hand? As described on the Information page under "Field Ore Conversion" section:
In a game, go near an ore patch that you want to convert. Mine an ore by hand if you do not have any in your inventory. Pick an ore stack in hand (coal and stone are also fine), then an UI button with an iron ore icon will appear on the top of screen. Please click on it to open the ore conversion GUI.
GUI button will show up in this way, as users do not need this on screen in the other 99% of the play time.
