To install Smewt on Linux, it is recommended to install its dependencies (especially PyQt4) through your distribution package manager.
On a Debian/Ubuntu system, you can install those with the following command:
sudo apt-get install python-pip python-qt4 python-cheetah python-lxml python-feedparser
Then just do the following:
sudo pip install smewt
Note: it is recommended to use pip instead of easy_install as the latter has some problems when installing Smewt in your applications menu.
You can install Smewt using the dmg provided here.
(Alternatively, you can also easy_install it if you feel more comfortable this way, but make sure you have a working version of PyQt4...)
You can install Smewt using the binary installer provided here.