I used apt-get to install cacti on my ubuntu 10.04 box. It said it installed correctly, but I'm not sure what port its listening on.
Answer
Try using http://localhost/cacti. None of the docs (that I have found, at least) reference a specific port, so I'd assume that it's listening on the default http port (80).
If nothing else, you can use netcat to scan your machine for open ports before and after launching cacti. Any ports that change between the two scans are probably in use by cacti.