Toughness cannot be directly modified. You have to buy powers or Advantages to raise your toughness. It's a core element of the rules.
The 'Trade-off' will be calculated automatically.
For example, if you have a PL 10 character, and you buy Protection 12 (assuming a base Toughness of +0) you'll find a set up up-down arrows next to the Toughness total, indicated a trade off has been made. If you try to buy your Defense higher than 8 an error message will be displayed at the top of the character sheet, showing that you have violated the tradeoff parameters.