When you 'Browse' for an application, you can select any executable file. Almost all applications have their executable file in the folder
/usr/bin, so navigate to there. Evolution, for example, has the executable file called 'evolution' in the
/usr/bin folder, so select this.
You can then edit the name for the launcher, the comment and the icon to use. Normally it is clever enough to choose the icon for you but if not, icons are generally stored in
Another way to find out the commands to use is to use the 'Main Menu' program which edits the Applications, Places, System menus for Ubuntu Classic (GNOME 2.x).
Once launched, navigate to the application you want to find the command for and click properties.
You can then look at the command for that application. You can use this command when creating your own launcher.
There is another method you can use for pre existing applications. Open up a file manager and navigate to
/usr/share/applications then right click on the launcher you want and select Copy to -> Desktop.