Neverworld works best with the Firestorm virtual world viewer, which you can download here.
If you already have Firestorm installed but are having problems connecting to Neverworld then you might have a version of Firestorm that only works with Second Life. This is possible because Firestorm comes in two different versions: one for Second Life and one for OpenSim. In order to tell which version you have, start the viewer and select the menu option Help > About Firestorm. The version number should include the words "with OpenSimulator support". If it doesn't then this is the Second Life version of Firestorm, and you need to download the OpenSim version of Firestorm.
Although we recommend using Firestorm, this isn't required. If you prefer, you can use any other OpenSim-compatible viewer.