try this code
Javascript:
<apex:column headerValue="Select">
<apex:inputcheckbox value="{!auth.IsSelected__c}" id="selectrecord" styleclass="isselected" onclick="disbutton();"/>
</apex:column>
<button type="button" style="display:none" onclick="invokeSaveNew(); return false;" id="saveButton">Add</button>
<script>
//first time check
var remember = document.getElementsByClassName('isselected');
if (remember.checked){
document.getElementById('saveButton').style.display = "block";
}
function disbutton()
{
var remember = document.getElementsByClassName('isselected');
if (remember.checked){
document.getElementById('saveButton').style.display = "block";
}else{
document.getElementById('saveButton').style.display = "none";
}
}
</script>
Also check if first time your checkbox is checked then the button is displayed.
Or if you are thinking to use stnadard button then use rerender. Let me know if you face any issue.