It shows me the homepage source code in the preview window, but not a preview of the document. i tried with pdf file and docx file.

Google Docs Viewer requires that your files be publicly accessible. Therefore, if you are using the walled garden setting, it might pose a problem. My development setup doesn't use the walled garden setting. If you do have this setting turned on, try turning it off and see if the preview works.

i have turned off. now i get a timestamp fatal error. i changed this entry in viewer.php line 4 to timezone, because i use php 5.2.9, but the same result. i reckon, it´s not enough, that i replace this one word, right? is it possible, you write your plugin also for php 5.2.9?

Ronald, I have rewritten the plugin to support PHP 5.2. See http://community.elgg.org/pg/plugins/thomasyung/read/874265?release=874777

Also, if you install the newer version, and you continue to get fatal exceptions related to DateTime, you might want to add the date.timezone setting to your php.ini.

Now it works excellent! Thank you very much! But only without walled garden. For the php.ini settings i will write a mail to my hoster. a query via info.php shows no value for date.timezone. very useful plugin for me.