I installed externaleditor.nbm v. 1.0 from the
Alpha update center, and accepted the default
option to install it into the NB user directory.
Under External Editor Settings, however, the lib
dir showed as "cannot find...". Looking at
ExtEdSettings, findElispDir neglects to consider
netbeans.user and only checks netbeans.home.
netbeans.user should be tried first, followed by
netbeans.home if nothing is found there.

Also the 'libdir' property should be of type File,
not String: I tried to set the lib directory
manually but found that clicking on "..." gave me
a string entry box rather than the expected
filechooser. Check

http://www.netbeans.org/download/dev/javadoc/OpenAPIs/org/openide/explorer/doc-files/api.html#core_editors_custom_parameters

for instructions on how to make the File property
editor in fact accept only directories with a
netbeans.el file beneath them, which would be nicest.