Problems with plugins

When i try to activate plugins i get this error:

Notice: has_cap was called with an argument that is deprecated since version 2.0! Usage of user levels by plugins and themes is deprecated. Use roles and capabilities instead. in /hsphere/local/home/svitz/blogg.andreas-svitzer.web.surftown.se/wp-includes/functions.php on line 3321

People are telling me that this is becouse the plugins are for older verisions of WP. But i tried alot of different plugins that all are
supposed to work for 3.0.1.

I really need help with this so i can finnish my pojekt.

Thanks for answears