When I active PluginsAdmin, I've this :
#0 /home/stat1/plugins/PluginsAdmin/PluginsAdmin.php(28): Piwik_Translate('PluginsAdmin_Me...')
#1 /home/stat1/core/PluginsManager.php(170): Piwik_PluginsAdmin->postLoad()
#2 /home/stat1/core/FrontController.php(238): Piwik_PluginsManager->postLoadPlugins()
#3 /home/stat1/index.php(51): Piwik_FrontController->init()
Are you using 0.28 or the tip of the trunk in svn?
What language are you using?
Sorry, I should have seen this. PluginsAdmin is from before the refactoring. (It's now CorePluginsAdmin.) Please install the latest version. Do not copy the files into your existing installation directory.
you can fix your existing piwik installation simply by following instructions in http://piwik.org/faq/update/#faq_6