The most likely cause of the "Could not determine the database schema. Check your connection string and try again." error is that the query builder is unable to establish a connection to the web service, and not necessarily an internal error in Query Builder. This could be due to a number of possible reasons:
Could you first double-check the URL? It is my understanding that you are working with qWSSP. If that is the case, and you installed it in the default location, you will need to change the text box that says "QdabraWebService" to say qWSSP, or another directory if you installed it in another location.
If that still does not work, please verify that the web service is running by opening a browser and going to the defaut page of the web service (usually http://[server name]/[installed directory] and clicking the top link ("Administration Tool"). If that opens successfully, it is a good sign that the service is running and that there aren't any security settings blocking you.