You can also click the Start button and drag an application, like konsole, down to the taskbar, and drop it. But I am affraid you might have lost the command for konsole, and that is why the icon is greyed out. Open another terminal and try typing "konsole" without the quotes. If it opens you just need to fix the link on the taskbar.
To fix the icon in the taskbar, right click on it and select Properties. Edit the Command in that box to read "konsole" without the quotes, and close it. Now the icon should work.