Unfortunately by some oversight, there is no directly link to the search feature on the default bbPress 0.9 templates (this is fixed in the next version – perhaps too prominently given how weak the built-in search is in bbPress).
You can make a link to search.php yourself in the header.php or footer.php to have it on all pages.
<a href="<?php bb_option('uri'); ?>search.php">Search</a>
Or you can make a mini-form that passes the field “search”
<form action="<?php bb_option('uri'); ?>search.php" method="get">
<input name="search" id="search"></form>