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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.
S1S Eval: Jesse's requests are reasonable.
[S1S-EVAL]
Sorry, but this is actually important, please fix it. Also check ${netbeans.dirs}: use StringTokenizer with File.pathSeparator as the delimiter and check all encountered directory names.
Issue #28683 suggests a more general solution.
I fixed this some time ago to use InstalledFileLocator. I just loaded nb3.5 and the externaleditor from autoupdate to my userdir and verified the fix.