The OpenCog codebase is available on GitHub
Along with the code, perusing the GitHub Issues there gives some information regarding what work has been done recently on the codebase, and some of what is planned for the near future.
Instructions for building the code are on the Building OpenCog page.
A rough indication of the maturity of different parts of the codebase is on the Code Maturity Guide page.
A series of tutorials on how to develop using the OpenCog framework can be found at the Hands On With OpenCog page.