found a fix at bgevolution.com for a bug in wordpress mu when enabling /disabling the plugins menu
As reported in my previous post, Wordpress MU has a bug where if you enable the Plugin menu, then disable it, you will not be able to re-enable it. To re-enable the Plugin menu, go into your PHPMyAdmin interface, [...]
found a fix at bgevolution.com for a bug in wordpress mu when enabling /disabling the plugins menu
As reported in my previous post, Wordpress MU has a bug where if you enable the Plugin menu, then disable it, you will not be able to re-enable it. To re-enable the Plugin menu, go into your PHPMyAdmin interface, open the database for your Wordpress MU site, open the wp_sitemeta table. Switch to the browser tab, rather than the structure tab. Locate the menu_items meta key. It should have an empty meta value, based on what I have observed, and what others are saying.
check it out :
http://www.bgevolution.com/blog/index.php/wordpress-mu-multi-user-plugin-menu-enabledisable-bug-workaround/