KDE Help Center can be called in several ways:
The most common will probably be from within an application. Choose Help->Contents to open that applications help file, at the contents page.
Choose the big K in your panel, and select Help to open KDE Help Center, starting at the default welcome page.
The Kicker panel contains by default an icon to call KDE Help Center. Again, the default welcome page is displayed.
KDE Help Center may be started using a URL to display a file. URLs have been added for Info and man pages also. You can use them as follows:
khelpcenter help:/kedit
Opens the Kedit help file, at the contents page.
khelpcenter file:/usr/local/src/qt/html/index.html
khelpcenter man:/strcpy
khelpcenter info:/gcc
Invoking khelpcenter with no parameters opens the default welcome page.