The internet pointed me to this page: http://puredata.info/docs/faq/debian which has very clear instructions expect that the necessary directory doesn't exist for my version of Ubuntu, 14.10. In particular, the procedure adds the following line to my apt/sources.list:
deb http://apt.puredata.info/releases utopic main
which tries to fetch:
But http://apt.puredata.info/releases/dists/utopic does not exist.
Searching through this forum I see some people using pd-extended on Ubuntu 14.10, so it must be possible.
How did they do it?