Got 500 error when I try to install or use plugins

[cazz]

Hi
I trying to use cacti to monitor my servers

I have use it Before to monitor some switchar but is first time I use plugins.

I have no problem to install monitor plugin but when I push the monitor button I get 500 error and dont say so much more then that
All I did was copy monitor folder to the plugin folder, that all, I maybe have to change some right?

After that I trying to use uptime but it show in the plugin management but when I try to install I got 500 error
Even there I just copy the folder to the plugin folder.

But at first I did edit the setup.php file, I find the information here
http://forums.cacti.net/viewtopic.php?p=176044#p176044

I just add the Three line at the top of the setup.php (after <?php tag)

I have no idea what more I can do?