First, you may need to take a look at this important information.

Now, once you've uploaded your drupal configuration files in your opensolr index, you should make sure that all the requestHandlers inside the solrconfig.xml file, will have the preceding slash at the begining of it's name.

For example:

<requesthandler name="standard">

should become

<requesthandler name="/standard">

Make sure you do the same for all other requestHandler tag in your solrconfig.xml file. Otherwise drupal will keep saying it can't find your opensolr server.

Another known issue is described here.

If you still have issues, please Contact Support.

We will be happy to set it all up for you.