To get started, install Git (http://code.google.com/p/git-osx-installer/), open Terminal and navigate to the parent folder you'd like to put papervision, e.g. your general actionscript workspace folder. (Hint: you can type 'cd ' then drag and drop the folder from a finder window onto the Terminal window to instantly get the path.)
Now enter the clone url listed at their Github page followed by the name of the new folder you'd like it to appear in, like so:
git clone git://github.com/Papervision3D/Papervision3D.git Papervision3D3
Instantly, it will create a directory called Papervision3D3, automatically git-initialize it, and pull the entire repository in a matter of seconds. That's right, using Git you get to work in the entire repository locally, create branches and so forth. Unlike SVN git works from a single .git folder within the main folder, instead of polluting all subfolders with .svn garbage -- you can even move the initialized folder wherever you like and it will still work fine.
You can't push any changes you make directly back to the official Papervision3D repository. To set up a public or private fork of their project get a Github account and click the button at their project that says "fork". Now you have a working copy at your own hosted page and can pull, commit, and push to your heart's content! Later it's easy to do diffs and merges with their project, and if you write something worth keeping they can do the same, easily bringing portions of code from around the community into their build selectively.
They call this "social coding".
Unless you have ambitious plans to contribute to their project right away though, to get started just do a clone as described above, which is more like checking something out of a googlecode public svn. You can import the folder into Flash Builder, Flex or FDT and run the file called Main to see a wireframe 3D example scene that already works. Go team!