As far as I can tell (By sheer guessing and spending 20 minutes looking through NEI. Why on Earth is there almost no useful information about how to do things in this mod?), the way you create concrete is by combining one of the many recipes for concrete into a Mixer, of which the one I have is a Ce…

Measuring Pot and click on the border area of the Mixing Bowl, not the center of it, this makes sure it can distinguish between “throw Container into Bowl” and “interact with Fluids to/from Bowl”.