If you need assistance during a tournament, you can call the Director directly from the playing screen. Click the blue box with three white horizontal bars at the top of the screen (the "hamburger menu") and select Call Director from the options.
To contact a Director after your table has finished and you're back in the lobby, you can send them a private chat. You can do this by clicking their username in the chat area or by searching for them in the People tab.
If you're playing in an ACBL game (such as a Speedball or Clubhouse tournament), go to the People tab, click Search, and type ACBL_. You'll see a list of Directors with usernames like ACBL_##. Those with a green dot are online and available to assist you.