Pax is being developed with Monodevelop on Linux and with the Windows version thereof, called Xamarin Studio. Visual Studio should be able to read the solution and project files but adjustments might be needed.
Getting the source and data
I use the fossil source code managment system, because it's simple and complete. This page and the tracker are both included with the software. If you install that program, checkout from
http://paxgame.sf.net/cgi-bin/fossil. Alternatively, there is a git mirror available at
git://git.code.sf.net/p/paxgame/mirror. Note that it lags a bit behind and commits to that repo are currently not merged back into fossil. You can also just download the latest source code as zip.
The default mod uses GNU wget and make to download data from sourceforge, to avoid having to include the larger files in the source code repository. If you don't want to install that software, you can manually download the data packages from sourceforge's file release mechanism. Then, imagemagick's convert is used to generate the textures from the data. In the future this dependency might be dropped and the textures generated by Pax on the first startup.
Windows x86 versions of the libraries are included. On other systems you need to install them.