Plugin-Page not available

Hi there,
I have been using Piwik for quite a while and am very happy with it!
Today I wanted to install a plugin. But when I tried to click in the administration area of the plugin list, Piwik logged me off and it appears again the log-in page.
Can someone tell me what is the problem?

Yours sincerely

You weren’t logged in as superuser.

We plan to clean up the navigation through the admin/super-user areas in 0.5.5.

okay, thanks a lot for your answer!