Installation on Ubuntu is straightforward when using APT on the Ubuntu management VM:
$ sudo apt-get install cacti
It will trigger a series of installation and setup steps, including the MySQL database, web server (Apache or lighttpd), and various configuration tasks. Once installed, navigate to http://<ip>/cacti to get started. The last step is to log in with the default username and password (admin/admin); you will be prompted to change the password.
Once you are logged in, you can follow the documentation to add a device and associate it with a template. There is a Cisco router premade template that you can go with. Cacti has good documentation on http://docs.cacti.net/ for adding a device and creating your first graph, so ...