Pd is included in both the main distro. To install, use apt-get install, the official packages are called puredata, gem, pd-zexy, pd-aubio, pd-csound, etc.

For pd-extended, you can use the apt repository. You can either add this line to the bottom of /etc/apt/sources.list Or you can add it in the GUI administration program Synaptic (in the Settings -> Repositories menu, then the Third-Party Software tab). Choose the line for your version of Debian or Ubuntu from the options below:

 # Debian/stable
 deb http://apt.puredata.info/releases stable main
 # Debian/testing
 deb http://apt.puredata.info/releases testing main
 # Debian/sarge
 deb http://apt.puredata.info/releases sarge main
 # Debian/etch
 deb http://apt.puredata.info/releases etch main
 # Debian/lenny
 deb http://apt.puredata.info/releases lenny main

Or Ubuntu:

 # Ubuntu/dapper 06.04 LTS
 deb http://apt.puredata.info/releases dapper main
 # Ubuntu/edgy 06.10
 deb http://apt.puredata.info/releases edgy main
 # Ubuntu/feisty 07.04
 deb http://apt.puredata.info/releases feisty main
 # Ubuntu/gutsy 07.10
 deb http://apt.puredata.info/releases gutsy main
 # Ubuntu/hardy 08.04 LTS
 deb http://apt.puredata.info/releases hardy main
 # Ubuntu/intrepid 08.10
 deb http://apt.puredata.info/releases intrepid main
 # Ubuntu/jaunty 09.04
 deb http://apt.puredata.info/releases jaunty main
 # Ubuntu/karmic 09.10
 deb http://apt.puredata.info/releases karmic main
 # Ubuntu/lucid 10.04 LTS  (NOT AVAILABLE YET)
 deb http://apt.puredata.info/releases lucid main