The webassist directory should have been copied from the install/webassist location to the root during the installation process... perhaps you installed on the live server and haven't done a get locally?
It appears there is a case sensitivity issue with that page that occurs a few places. The easiest way to fix it is do do a find and replace using dreamweaver on "your entire local site" and find "options_Add.php" and replace with "options_add.php".