Hi Don,
Welcome to the forums!
Few versions ago the default location of the Find feature in Microsoft Streets and Trips depended on Windows keyboard and/or language settings. So you would go to Control Panel to change for example the default US English to Canadian English if you wanted the Find feature to default to Canada. This however I suspect is no longer the case, as I haven't heard people having issues with this in recent years. To the best of my knowledge now Streets & Trips just simply remembers the location of your most recent successful search. Try to locate any USA address using the Find dialog box, "OK" the address so a pushpin is created (or a Stop is created), and then close Streets & Trips. When you subsequently start Streets & Trips I do believe the Find dialog box should default to USA.