The property you are looking for is ".state" which returns the state of the checkbox:

Code
on *:dialog:xsys:sclick:4:{
  if ($did($dname,4).state == 1) { .echo -a 065 Activado }
  else { .echo -a 061 Desactivado }
}
Other observations:
You should have brackets () around the if statement comparison, followed by { } for the commands it triggers.
The "==" should have spaces around it to separate it from all other parts of the comparison text.