Make sure port tcp port 8080 is open in your Security group, that might be your problem. Then follow these steps:
Getting started with Cacti
Open TCP port 8080 (instructions for opening ports are above in the „Testing your Instance‟ section.
GoTo URL: http://[public-domain-name]:8080/cacti
Login as:
User Name: admin
Password: password
In the left hand column under the „Management‟ section click the link for „Devices‟
On the „Management: Devices” page click the „Localhost‟ device in the „Description‟ column.
Uncheck the „Disable Host‟ setting in the „General Host Options‟ section of the page. Then click the „Save‟ button at the bottom of the page.
Your server is now collecting data and can be viewed on the „Graph‟ tab. Because Cacti only collects data once every 60 seconds it will take a few minutes before your data will appear in the graphs.
Richard