activate button script

<script type="text/javascript">
function check_disabled(checked, next) {
  var next_step = document.getElementById(next);
  next_step.disabled = !checked;
}
</script>

<form name="frm" action="whatever.htm" method="post">
<input type="checkbox" name="check" onClick="check_disabled(this.checked, 'submit_id');" value="1" /> ik ga akkoord<br />
<input type="submit" name="submit" id="submit_id" value="naar volgende pagina" disabled="disabled" />
</form>

<script type="text/javascript">
function check_disabled(checked, next) {
  var next_step = document.getElementById(next);
  next_step.disabled = !checked;
}
</script>
 
<form name="frm" action="whatever.htm" method="post">
<input type="checkbox" name="check" onClick="check_disabled(this.checked, 'submit_id');" value="1" /> ik ga akkoord<br />
<input type="submit" name="submit" id="submit_id" value="naar volgende pagina" disabled="disabled" />
</form>