Sometimes it is not convenient to find packages through the main package window. In these cases, you can use the find window. Just choose the kind of search you want from the option menu, enter your search term in the entry field and click the find button. The results will be displayed in the list in the center of the window. You can then query, uninstall or verify those packages like you could in the main window.
As an example, say you wanted to find all the programs on your system that required the old gtk+-1.0. All these programs link with libgtk.so.1, and hence require it. You could set the search type to "require", and the search term to libgtk.so.1. The results of this search would be all packages that require gtk+-1.0. You could then select them all and query them to find more information about the individual packages or just uninstall them.