The checkbox() method of the FormBuilder class accepts the following values:
string $name
mixed $value
bool $checked
array $options
So, you need to edit your code as follows to assign the 'form-control' class:
{{ Fome::checkbox('K_Stenge_Prosedyre', 1, null, array('class' => 'form-control')) }}
Within the $options array, you can specify multiple classes or attributes. That said, you'll probably want to assign a custom class to your checkbox and manipulate it using CSS 3 as suggested in the link above.