Just make sure that you have uploaded the solr config files that your drupal module has provided for you.
To upload your files, simply head on to your OpenSolr control panel, click on your index name, and then click on the Config Files Editor tab.
Once there, you should be able to see a dialog like this:
NOTE: Your files have to be inside of a valid ZIP archive.
After the upload, you will receive upload confirmation messages for each file inside your ZIp archive.
In case of an error, you wil receive the full java error stack-trace, so that you can easily identify the errors inside your configuration files.
We recommend you use a JSON Prettyfier extension for your browser. Preferably you could use one of chrome's extenstions for that.