There are several ways to add an application to the panel:
Using the panel menu
Perhaps the easiest method is to use the entry "Panel | Add application" in the panel menu. (This is the menu that appears when pressing the K button on the panel). The menu shows all installed KDE applications. To add a button to the menu, simply click on the application.
Using the filemanager
You can also simply drag a kdelnk file from a window of the filemanager to the panel.