Press F2. A Browse window will appear. Choose a TXT file. If it's a valid position file, then animtools will switch the characters into the new position.
You can also setup shortcuts (to load your favorite positions when a certain hotkey is pressed) or bind them to buttons on the GUI. Dialogue writers can use scripting in order to switch automatically between positions when an appropriate "trigger" event has occured. But if you're testing out a newly-downloaded file then you'll probably be reliant on the F2 key.