In the Requirements Chapter of this install document you downloaded a zip file that contained the i2b2 core source code. This zip file contains all the i2b2 code needed to install the i2b2 cells including the i2b2 Server-Common.
Steps to extract the code
You will want to make note of the location as it will be needed further along in the i2b2 installation. In the installation document whenever you see YOUR_I2B2_SRC_DIR it will need to be replaced with the location of your i2b2 Core Source code.
For your convenience an installation Worksheet to track the setup locations has been provided in the Appendices Chapter.