After downloading the Salesforce.Com Connector for Outlook I noticed that it was hardcoded to what SFDC server I could connect to. I needed to do some testing and struggled to find where I could change the URL.  My end goal was to change the Server URL to point to https://test.salesforce.com/services/soap/c/10.0

After poking digging around in the registry I found a place that this could be changed. If you open your registry and go to “HKEY_CURRENT_USER/Software/Salesforce.Com/SM” you will see an entry called “Server URL.” There you can change that link to point to your sandbox instance. Once you have done that you can re-open Outlook and type in your login credentials. Make sure that when you login you also add your Salesforce.Com token to your password.  Hope this helps you save some time!