The Short Version
The Long Version
Our code is hosted on GitHub. Besides hosting and serving our code for us (thanks GitHub!), they provide a bunch of useful addons - tracking of forks, simple merge procedures, and stats tracking to name a few. It's not necessary that you use an account on github to develop Sirikata, but you might want to check it out. Especially if you want to do some experimental development or just work outside the main tree for awhile, its a nice way to easily track the mainline fork. Overall Git is a great a distributed version control system but it takes some getting used to so you may want to check out some tutorials.
Building Sirikata can be a complicated affair: it's a large piece of complex, platform independent software which leverages a large number of existing packages. Even worse, we currently have a limited number of test systems, generally those that the developers are working on. We would very much appreciate reports of any problems sent to the developer list, or even better would be fixes.
For reference, the current core developers have Linux, Mac and Windows machines. Most heavily favor Linux, especially the Ubuntu distribution.
Developer Preview (Prototype) Code
The current incarnation of Sirikata has its origins in a 6 months old prototype. This is available as a developer preview. Only the curious and brave will want to check this out; building it is more difficult than Sirikata.