imposm.parser runs with Python 2.5, 2.6 and 2.7. You need the install the multiprocessing package if you run Python 2.5.
The PBF parser is written as a C extension and you need to have a C/C++ compiler, the Python libraries and Google Protobuf.
On Ubuntu:
sudo aptitude install build-essential python-devel protobuf-compiler
You can install imposm.parser with pip or easy_install.
pip install imposm.parser
pip install path_or_url_to/imposm.parser-0.x.x.tar.gz