Pypy doesn't need a different kernel, it can just use ipykernel. It thus isn't listed separately in the list of Jupyter kernels.
You can use the same methods used for installing kernels for different Python environments to install a kernel for pypy. Eg,
pypy -m pip install ipykernel
pypy -m ipykernel install --user --name pypy --display-name "PyPy"
Or
pypy3 -m pip install ipykernel
pypy3 -m ipykernel install --user --name pypy3 --display-name "PyPy3"
The extra options are just there so that there are useful names for the kernels.
Note, however, that iPython 6 does not support Python 2.7, so if you aren't using PyPy3, you'd need to install iPython 5 in PyPy (I think it should still work with the most recent Jupyter, however).