The BabbleSim project consists of many components, most of them optional and each in its own separate git repository. The easiest and recommended way to fetch them is by using Androids repo
mkdir ~/bsim/ && cd ~/bsim/ repo init -u https://github.com/BabbleSim/manifest.git -m everything.xml -b master repo sync
#replace those paths as necessary
For a list and description of the provided manifests, see the manifest repository documenation
You may fetch each git repository manually or by other means. In this case, consult the folder structure page for information about how to place them.