It shows up immediatly, but it's not automatic. You have to select the planet the the resource converter is on and select the "convert resources" button. That takes you to a screen where you can enter how many of what type to convert to what you want.
Geo
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Try again and check very well the options... the first time that I have used the converted, I have converted Mineral to Mineral, and lost 30% of resources in the process!!!!!
You need to select the resource that you want to convert, and the final resource.