i don't have any problems on this machine to load externals, although both the debian package and devel are installed, so i think, it _is_ a user bug (sorry) or a build problem (flamewar) ...
agree..3 times ive been unable to load externals and each time it was due to incorrect user settings or build problems: specifying an audio interface that wasnt compiled in (build issue) building pd with a different compiler than the externals (build issue) and pd getting confused by the commented items in .pdrc (branch desynch issue)