Simple!, open user preferences > go to addons tab> click in Install from file> find the zip file> and click install.

Will appear a checkbox with the addon name, now just enable it and save the user preferences.

It doesnt work always, as any algorithm has a weak point this has too.

I cant say it will solve all your problems and I feel that I must say when it will fail...

* Spikes: and thin rods: Small spikes or small cilindricall features like branches on a tree will be lost if your resolution is not high enough. generally is some cases these features are just too small to consider putting the resolution that high, this addon works better with round and fat shapes rather than sharp and complex.

*Low resolution meshes: if your source mesh is too low poly the solver will not be able to compute the aproximate voronoi diagram that is used to connect the final vertices, but you can solve this by ssubdividing the mesh.

*Animated characters: At the risk of sounding repetitive... There's no such thing as Auto-Retopology This remesher wont create meshes good enough for animation if you want a good tool for Retopology I recomend retopoflow or even the built-in tools in blender because it should be hand-made.