Tutorial
How to Add a Search box for the Forum on a Nav Panel

Edit the Discussion Forum Category
Select the Discussion Forum from the drop down menu under Content Categories Edit Category
Script for this Category
Copy and paste the code below into the 'Script for this Category' box which can be found under the Category Script section.
<form name="frmSearch" method="post" action="/members/cfmbb/search.cfm"><input type="text" maxlength="100" size=10 value="" name="searchquery"/><input type="hidden" name="searchtype" value="any"><input type="submit" class="button" value="Search" name="search"/></form>