Do you really mean install? Eclipse Plugins self-install. Take the zip
file that contains your copy of JUnit and unzip it into the plugins
directory (for me that was C:\eclipse\plugins so I ended with
directory:
C:\eclipse\plugins\org.junit_3.8.1

Now stop (if necessary) and restart eclipse. It automatically scans
the plugins directory for plugins and loads them. You can see what
plugins it has loaded by opening the About window and clicking the
plugins button. It is normally a long list because most of the eclipse
internals are deployed as plugins.

That should be all you need. If you want to actually use it you can
use the eclipse File->New ... to create a JUnit test skeleton, then
write your test code, then use 'Run As' and pick JUnit. There isn't
much to it (once you know, of course).

Roger