To activate cheats open the ingame console window and enter them there.
By default pressing the ~ (tilde) key opens the console.

If that doesn't work you can rebind the key:

• Go to options
  
• Select controls
  
• Scroll down to other controls
  
• Rebind the console entry to your desired key

Technically, you can also enter cheats via the chat window but it doesn't always work. Entering them via the console should always work.