I was trying to figure out how to add applications to bottom panel. I know how to add them to the Plasma Desktop Folder, but didn't know how to add them to the bottom panel and I started clicking away and the more I clicked the more I messed up my desktop as seen in first screenshot.
To get back to fresh default panel:
at login click
Ctrl + Alt + F1 and enter user name and password and then at prompt:
it will confirm and ask "remove regular file /home/(you)/.kde4/share/config/plasma-desktop-appletsrc'?"
enter "yes" and click enter.
click Ctlrl + D to go back to log-in, then
click Ctrl + Alt + F7 to get back to graphical log-in.
A default plasma-desktop-appletsrc file will be regenerated of original widget layout and wallpaper.
Your desktop should be back to original state as is shown in second screen shot.
To place application in panel, right click on Kickoff Launcher icon in far left and select menu editor. Then choose application you want and drag and drop to panel. Unless there is simpler way to do this, but this did it for me.
Cheers, Lisa Marie
Edited, as I forgot the "Ctrl + D" step.