A binary distribution of JOpera for Eclipse is currently available.
The latest version of JOpera is 2.3.7
To download and install JOpera, just point your Eclipse to the following update site:
http://www.update.jopera.org
JOpera 2.3.0 and above runs on Eclipse 3.3 Europa/3.4 Ganymede on Java 1.5/1.6.
JOpera 2.1.2 runs on Eclipse 3.2.1 with GEF 3.2 on Java 1.4 (not 1.5).
The new Web Monitor only supports Firefox 2.0.
Previous versions of JOpera (1.15) require Eclipse 3.1.1, also on Java 1.4.
Note: After adding the JOpera update site URL it may be necessary to close and reopen the new Update Manager dialog before proceeding with the installation of JOpera on Eclipse 3.4 Ganymede.
Make sure you do a full rebuild of your JOpera projects after upgrading to a new version or switching to a different version of Java.

To keep improving JOpera, we are thankful for your feedback!
It seems that the JOpera update site listed above is blocked from the great firewall of China. Let us know in detail if you have trouble accessing it so that we can try to find a solution.