The ECW plugin requires four libraries to work:
The ermapper.jar file which should be in the /lib/ folder of openjump.
Then the following three dll files should be in /lib/ext/ :
NCScnet.dll
NCSEcw.dll
NCSUtil.dll

if ecw files can be loaded (i.e. the extension is displayed in 
the file choser) but the file is not displayed, then it may be useful 
to add the following JAVA VM argument: 
-Djava.library.path=C:\progams\openjump\lib\ext