Field collapsing classloading issues

I applied the SOLR field collapsing patch to solr 3.1.0. I'm not really sure about what to add in solrconfig.xml. Right now I've added the following :

Applying SOLR-236 field collapse patch to Solr 3.1.0

I've been trying to install the field collapse patch to solr 3.1.0 using the following link :https://issues.apache.org/jira/browse/SOLR-236However, I'm not entirely sure which patch to download. How do I decide on that?Also, as I understand it, I have to cd into the apache-solr-3.1.0 directory