From any menu
To add a launcher to a panel from a menu, perform one of the following steps:
Open a menu that contains the launcher. Drag the launcher on to the panel.
Open the menu that contains the launcher and right-click on the title of the launcher. Choose Add this launcher to panel. This method will only work if the launcher is on a sub-menu of the menu that you opened.
From the file manager
To add a launcher to a panel from the file manager, find the .desktop file for the launcher in your file system, then drag the .desktop file to the panel.
i.e. your desktop files will be located in