If you want to use the solved tick you first have to get that arranged by an admin for your subcategory “United States” in general like it was done (on explicit application) for the subcategory “Deutschland (Germany)”.
If that is done you will see the “solved” checkbox in front of every single topic in your subcat topic lists. And then the originator (only himself) of a topic can set his topic to solved. According to my knowledge there is no more detailed function of the solved flag (like setting solved to a single post) available.
We entered a short description in our German user guide - “Lösung” (readable in english via translation button).