The page contains information about how to install the latest version of Bempp (bempp-cl 0.1.1). You can find information about installing the previous version (Bempp 3.3.4) here.
The easiest way to install bempp-cl is to use pip or conda:
pip install bempp-cl conda install bempp-cl
More detailed installation instuctions can be found here.
You can find the source code of bempp-cl on GitHub.