Workers are specific to their contraption, which means you can't put a Mill worker in the Dairy, or a Bakery Worker in the Mill, etc.
The only workers you can put in the Pasture are cows (and Doobicorn!), and they'll show up at the Bus Stop just like the other workers!
Look at the icon on the worker's info screen -- It shows which contraption (Dairy, Mill, Bakery, or Pasture) they can go in, and lets you know if putting them in that contraption will increase or decrease the contraption's productivity.
Everything you need to know about getting new workers can be found here: How do I get (more) WORKERS?
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