How do I add a class to the input box on the search sidebox?

in the file...
includes/templates/yourtemplate/sideboxes/tpl_search.php

change...

$content .= zen_draw_input_field('keyword', '', 'size="18" maxlength="100" style="width: '

to..

$content .= zen_draw_input_field('keyword', '', 'size="18" class="searchinput" maxlength="100" style="width: '