or escape the single quotes around Add.php. The problem is when you hit the second single qt right in front of Add.php, you are ending the string, so php sees the string as:
'<input type="button" name="Submit3" value="Add New" onClick="document.location.href=' and the rest as garbage.
echo '<input type="button" name="Submit3" value="Add New" onClick="document.location.href=\'Add.php\'">';