The "Search Catalog" hint text is from catalogbrowse.xml under USM_HOME\view\webapps\usm\locale\icusen\request\ .
do following to customizing catalogbrowse.xml to change the hint word "Search Catalog" to what you would like :
1) locate catalogbrowse.xml under USM_HOME\view\webapps\usm\locale\icusen\request\
2) copy this catalogbrowse.xml and copy it into USM_HOME\filestore\custom\locale\icusen\request\ ( Note : if you already has customized catalogbrowse.xml in this folder , then you can just add the new modification into the exisiting customized catalogbrowse.xml here ).
3) then modify the catalogbrowse.xml under USM_HOME\filestore\custom\locale\icusen\request\ :
to what you want
4) stop catalog service
5) go to USM_HOME\view\translets\ folder and delete all those translet files under this folder
6) restart catalog service