First click on the Sign Up page link and register your avatar.
The recommended viewer is FIRESTORM VIEWER for OpenSim. Make sure it is compatible with your pc.
After downloading your viewer; start it up and click on preference then click on opensim.
Where it says grid uri place the following: login.aviworlds.com:8002 then click apply and OK. See example picture below
Make sure you have the correct user name and password.