Hi John,
This seems to be a macro and not a plugin.
The FibrilTool macro is in this file:
http://www.nature.com/nprot/journal/v9/n2/extref/nprot.2014.024-S3.txtOpen a new Text window in ImageJ (Ctrl+Shift+N).
Copy the macro in this text window and install this macro (Ctrl+I)
The Fibril Tool icon appears in the ImageJ menu.
Success!
Peter