Edit in JSFiddle

$('#check').change(function(){
	if($(this).attr('checked')) {
		$('#submit').attr('disabled',false);
	} else {
		$('#submit').attr('disabled',true);
	}
});

<div class="wrap">
<input type="checkbox" name="agreement" id="check"><label for="check">チェック！</label><br>
<input type="submit" id="submit" value="送信" disabled="disabled">
</div>

.wrap {
    width:120px;
    margin:30px auto;
}