The graphviz conda package is no Python package. It simply puts the graphviz files into your virtual env's Library/ directory. Look e.g. for dot.exe in the Library/bin/ directory.

~~To install the graphviz Python package, you can use pip: conda install pip and pip install graphviz. Always prefer conda packages if they are available over pip packages. Search for the package you need (conda search pkgxy) and then install it (conda install pkgxy). If it is not available, you can always build your own conda packages or you can try for user-built packages.~~

Update : There exists now a python-graphviz package at which contains the Python interface for the graphviz tool. Simply install it with conda install python-graphviz.
(Thanks to wedran and g-kaklam for posting this solution and to endolith for notifying me).