Toad for Data Analyst is trying to promote and assist an Analyst workflow of Understand, Query, Report and Automate. The application default configuration organizes the windows by hiding the windows that are associated with those four groups. If you were to click on the Understand button at the top of the app (see attacged WorkflowButtons.jpg) you would only see the ER Diagram or Database Browser windows. Under the Query button you would only see the Query Builder or Editor windows.
When you push on the editor tool button as you show, it will always open a new empty editor. To navigate in this configuration you need to push the Query workflow button.
You may feel more comfortable with a different configuration. Go to the Tools|Configuration Wizard and choose any of the packaged configurations or customize the default. Try the MS Management Studio one since you are connecting to SQl Server. Or using the customize section deselecte the Hide documents per group.
I think this will clear things up.