The default location for these files would the path listed (ex. C:\Program Files\Apache Software Foundation\solr\conf). In this folder you should also see file like solrconfig.xml, schema.xml, etc. This file is in the directory by default. If for some reason its not there you can add it. I've attached one that you can review.
Is your Solr instance running? If so you can browse to the admin page (ex. http://localhost:8080/solr/admin) and on the this page look at the value at the top for SolrHome.