Remember that category menus and search boxes are all just result restriction tools and routes to content, things users use to acheive goals on your site.
Assuming it is actually visible and sensibly placed, users might be using the search but fiding it fails to help them acheive their goals as quickly as they can with the category search. No amount of focus tricks like bright colours will help here, in fact it will be a hinderence if the search function is poor.
If they are able to find things more quickly using your category search why attempt to force them over to search box?
Ask my users why they choose the category over the search or why they choose not to use the search.
Then use my resopurces to improve the prefered method and if the users are put off the search because of its poor functionality then improve the search function.
If they just find it easier to use the categories then let them.
The best way to focus their attention is by making sure it is useful to them and helps them acheive goals with a minimum of fuss.