Well, there's nothing to do with "execute it instead of include it", because when a file is included, it is executed (it wouldn't be very useful, otherwise).
Juding from the error message, it sounds like PHP can't find the kalendar files.
Seeing as you appear to be on a Windows machine, shouldn't you be using "\" in your file path instead of "/"?
Actually, because "\" has a special meaning in strings, that should be "\":
include("modules\kalendar1\index.php");
As the error message implies, it will look for [directory where this script is]\modules\kalendar1\index.php first. If it doesn't find it, it will next look for c:\apache\php\pear\modules\kalendar1\index.php.
If it still complains, make sure the file's permissions allow it to be read by the server (generally, this means that everyone should be able to read it.)