Well then, step-by-step:

• Create a private variable of type QString in your header file in your class
  
  Code:

  QString myString;

• In your constructor, set a default value to this variable.
  
  Code:

  myString = "mydefaultstuff"

• In the designer, right click the combobox, click "go to slot...", and select the slot you've found in the doc
• in the generated method, assign your variable - set its value to the one that is returned by the slot
  
  Code:

  myString = text;

• In your button's clicked(), use the value of this variable.

Note that there should be some function which returns the current item in the combo box - using that in your clicked() should eliminate the need of introducing the variable.