You can get the released version using your favorite Python package manager:
easy_install mahotas
or:
pip install mahotas
If you prefer, you can download the source from PyPI and run:
python setup.py install
You will need to have numpy and a C++ compiler.
On Windows, Christoph Gohlke does an excelent job maintaining binary packages of mahotas (and several other packages).
If you use Python(x, y), which is often a good solution, you can find mahotas in the Additional Plugins page.
Mahotas is available for FreeBSD as graphics/mahotas.
For Macports, mahotas is available as py27-mahotas.
Mahotas is available as python-mahotas.