You can always right-click in the Servers view to bring up the New Server dialog. That will allow you to add a new server to STS. You can also double-click on an existing server in the view to bring up a configuration editor. From there you can edit several properties. STS will also put a Servers project in your Project Explorer/Package Explorer. That project will contain all the .xml and .properties files for the server if you want to edit those directly.
Leo Dos Santos
Committer, SpringSource Tool Suite