Hey, no worries, thanks for playing! Once you've collected a new barrel you interact with the Slime Collection Station again (with the valve attached) and it'll place the barrel. Then you spin the valve shut and it'll start collecting again.
Thanks for the feedback about the numpad, you're right I hadn't considered laptops and things, it just mapped nicely with the analogue-style machines we were building, but will make sure I add secondary options for the numbers across the top in future projects