We offer two REX Solr Connectors, one for RLP (native), and one for REX-JE. The article Enhancing Apache Solr with Entity Extraction describes the RLP (native) product. As of RLP 7.12.1, instructions can be found in the Application Developer’s Guide, chapter 14.
The connector for REX-JE currently (as of May 2015) is an alpha product, available only as a special shipment. It has been tested with REX-JE 22.214.171.124. Talk to a Basis rep to receive a copy.