The dRS version corresponds to the db4o version. Ensure that the db4o and the dRS version are the same.
For a simple db4o to db4o replication scenario you will only need the add the dRS-X.XX-core.jar. to the classpath.
The RDBMS Replication needs dRS, Hibernate and its dependencies. You need to add the following jars to the classpath:
Please, refer to the Hibernate documentation for an explanation of these dependencies.
If you are going to use dRS with HSQLDB you will need hsqldb-1.8.0.7.jar in addition to Hibernate.