The driver encountered an error: cannot load Java class

Hi,

I am running RadRails build: 2.0.5.1278709071.

I am trying to learn how to user Ruby on Rails. I have set up a project,
that generates files to link to an Oracle Express 11g database.

When select the project, and then run: Rake -> db -> migrate, I get the
following error message:

rake aborted!
The driver encountered an error: cannot load Java class
oracle.jdbc.driver.OracleDriver

Can anyone help?

For example, what files do I need to download from Oracle, and where
should I place them on my computer?

Many thanks,

Sean