I never used cacti,but since nobody replied so far...
I searched a little.You should visit their doc page here
Point your web browser to: http://your-server/cacti with a username/password of admin. You will be required to change this password immediately. Make sure to fill in all of the path variables carefully and correctly on the following screen.
That's why there's no cacti in the menu,because you log into it with your web browser.