Because that is not forms[0]. You have another form above it.
You also have a tag <form name="form2"> in your page. Get rid of it.
Your link should say:
javascript:document.forms[1].submit();""
or
javascript:document.searchclas.submit();""
(you are using a form name, why not refer to it?)
Hope that helps...
<edit>
that should be "javascript" without the space... PHPBuilder is adding the space, probably to prevent hacking
</edit>