There are several ways of accomplishing preselection of a row in
STRUTS. It looks like the option you have chosen is to loop through your Collection yourself and build your select box manually. In this case you have to place the conditional statement, which I believe is another logic tag, that tests whether the value you'd like to preselect has been reached in the loop, and if so you'll have to output the option line with the selected attribute set.
Other options are using the the html:options or html:optionsCollection STRUTS tags and using their api to preselect the value for you.