The search box is in the Upper right center of whatever board page you are on.
Enter your search term in the first box (Search...).
If you want to search that board click the magnifying glass symbol.
If you want to search another board type in the first letter or three of the board you want to search, then click the symbol.
If you want to search all boards, empty the second box and click the symbol.
To search Chow, click the "Take me to Chow" button and use the search box there.