Running Active Commerce on a Sitecore Solr implementation requires upgrading the xActiveCommerce.ContentSearch.config (which contains product-specific indexes and configuration) to use the Solr provider.
We've attached a couple examples here. One is for Active Commerce 3.1 on Sitecore 7.2 (xActiveCommerce.ContentSearch.Solr.config.AC31SC72.example) and another for Active Commerce 3.2 on Sitecore 7.5 (xActiveCommerce.ContentSearch.Solr.config.AC32SC75.example).
To enable, you would rename (adding a ".example") or delete the Lucene version of the file (xActiveCommerce.ContentSearch.config) and drop in the Solr version (removing the ".ACxxSCxx.example" to enable).
Please note these are provided as-is. Please adjust for your implementation and test accordingly.
At this time it can require significant custom work to get the Active Commerce faceted filtering to function properly with Solr, due to differences in Sitecore's crawling and searching implementations with Solr. Active Commerce cannot currently provide support for our out-of-the-box faceted filtering on Solr.