To join Collaborate Classic sessions in Ubuntu, you’ll need to install OpenJDK 7 and some extra packages
Open up the Ubuntu Software Centre. Search for, and install the following:
- OpenJDK Java 7 Runtime
- IcedTea Java Web Start
- IcedTea Java PlugIn
If you have other java runtimes installed, you will need to set OpenJDK as the default (see step two at that link).