Snow Leopard already has Java installed on it, so it's pretty straightforward installing Apache Solr on Mac OS X versions 10.6 and above.

For Solr 3.5.0 on drupal 7

Download Solr-3.5 from the Apache Archive here
Download the apachesolr module from https://drupal.org/project/apachesolr

Assuming the above folders are in the Downloads folder:

Copy the solr-3.x config files from the module to the apache solr config cp ~/Downloads/apachesolr/solr-conf/solr-3.x/* ~/Downloads/apache-solr-3.5.0/example/solr/conf

CD into the example directory cd ~/Downloads/apache-solr-3.5.0/example/

Start the Solr server java -jar start.jar

To install Apache Solr on Tomcat in a Mac environment, see http://drupal.org/node/714642