Although I guess it's nice to know different methods for customizing your desktop, the "right click and choose an app from the dialog" is really not the simplest way.
All you have to do is open the menu (for example Applications -> Accessories), left-click on the app you want in the panel, then drag it over to the panel and drop it. Voila! The only reason to use the Add to Panel dialog (IMO) is if you want to add an applet (that doesn't have a separate menu item) or you want to add a custom launcher to the panel.
Similarly, you can rearrange launchers on the panel by just dragging them around with the middle mouse button. Note that many of the preset launchers and applets are locked in place by default so before you can move them for the first time you have to right-click and de-select "Lock to Panel".
Simplifications
Although I guess it's nice to know different methods for customizing your desktop, the "right click and choose an app from the dialog" is really not the simplest way.
All you have to do is open the menu (for example Applications -> Accessories), left-click on the app you want in the panel, then drag it over to the panel and drop it. Voila! The only reason to use the Add to Panel dialog (IMO) is if you want to add an applet (that doesn't have a separate menu item) or you want to add a custom launcher to the panel.
Similarly, you can rearrange launchers on the panel by just dragging them around with the middle mouse button. Note that many of the preset launchers and applets are locked in place by default so before you can move them for the first time you have to right-click and de-select "Lock to Panel".
Cheers!