The command is
/etc/init.d/nessusd start to start the service (sometimes already started depending on config)

You then need to open your web browser and point it at

https://localhost:8834 to access the GUI

Make sure you remember the user account you setup during install and config otherwise you have to run (path may vary on your system depending on your install)

/opt/nessus/sbin/nessus -adduser

Not that i've had to do that myself, errrr!

You should also run

/opt/nessus/sbin/nessus -update-plugins

every few days to keep it up to date