Press Run, or Ctrl+Enter, to run the program: its reports appear here. New to Probl? The explains the language, and the lists what's built in.
Hover a name in the editor to see what it is, and Cmd-click or Ctrl-click it (or press F12) to go to where it's declared, or to its entry here. F2 selects a name everywhere it's used, so typing renames it. Completion comes as you type, or with Ctrl-Space, and a call's signature shows while you type its arguments.