He meant that you can't do this:
Qt Code: 
  1. void Button::apasat()
  2. {
  3. MyWidget::verifica(y,x);
  4. }
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You need to create an object of type MyWidget, something like this:
Qt Code: 
  1. MyWidget obj; //create an instance of MyWidget class
  2. obj.verifica(); // run the member function;
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An if MyWidget is an GUI widget you might want to allocate memory on the heap for objects, like this:
Qt Code: 
  1. MyWidget *ptr = new MyWidget(parent); //alocate with "new" and give a parent
  2. ptr->verifica(); // call member function with pointer
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If you don't understand take at least a tutorial of c++ first, and than a Qt one (or Qt examples), but i strongly recommend a book.