I'm using the zenpage default theme and I want to modify the side bar menu with some new links (guestbook, imprint), so they will open at the left side like the standard links. e.g. "All news" or "Contact us" - how can I do this ?

Add the links on the `sidebar.php` page of the theme. how the pages that you are opening look like is a matter of theming if they are not part of the Zenphoto theme.

I suggest to read the theming tutorial to get familar with it.

Ok, I read the tutorial, but I dont understand how to add a link in the correct way. Now I have a link, but not the function to open the content in "content-left" area. I mean, if you click the link the burning book will appear like a gallery (or news content) on the left side. The link(s) should not be under "Pages", there is a category like "Other Stuff" with Guestbook, Imprint and so on.

Sorry, my english is very bad - there is not only php, I have to learn *g*

As I said if the pages you want to open are not part of the Zenphoto theme, as a guestbook will be since there is no guestbook plugin for Zenphoto. You generally can't open a separate script like your guestbook within a Zenphoto theme page. That will probably require to style the theme of the guestbook like the theme of zenphoto.

While it might be possible to imbeed that guestbook somehow this is something I can't answer without knowing that guestbook. Anyway that would most likely exceed the limits of free support anyway.

While I am actually also German I can't make an exception as our forum language is simply English.

Sorry, if that sounds harsh, but you probably need to learn a few things about web techniques before proceeding.

"... I can't make an exception as our forum language is simply English"
Thats the reason why I wrote in English - have I asked for a german translation ? No.
I asked for some help.
"Sorry, if that sounds harsh, but you probably need to learn a few things about web techniques before proceeding."
The best answer anywhere ... by the way - thanks for your time.
Ok, somehow it will go.

I hope we did not get any misunderstandings as we all know that happens easily in written text on the net? You are welcome with help requests but special needs like integrated/combining other scripts like a guestbook is not exactly Zenphoto specific.

I just mentioned the German vs English as I did made exceptions in the past which did no good actually.

It's ok - if you will answer to all questions explicitly, you have to do a full-time-job. For now I did it with iframes in a new page. Not perfect, but it works *g*
There is a lot to do, until the site lookes like my idea.
But it's a lot of fun, too.