It will not work with a JVM version inferior to 7.0.
If you are looking for an old version of OcaIDE, you can grab the jar from here and install it manually, or recompile it from source.
Also, beware that the plug-in has been reported not to work with GCJ (due to incomplete Java support in GCJ)
It works on Windows, Linux and Mac OS X. Some features are unavailable under Windows (debugger checkpoints and interrupting the toplevel, mainly).
Enter http://www.algo-prog.info/ocaide/ as the update site.
Check the OcaIDE category or feature in the list.
Then, click Next twice, accept the agreement and click Finish.
The plug-in then downloads... It can take a while depending on your connection speed.
Accept the installation (the plug-in is not digitally signed), and restart Eclipse when it asks you to.
As soon as Eclipse is restarted, you can start using the plug-in.
You can access the online help by clicking on Help > Help Contents in the Eclipse main menu, and choosing OCaml Development User Guide in the list of available topics.