Go to your admin (path should be something like) -
http://www.yourdomain.com/cgi-bin/dbman/admin/admin.cgi?
Go to Paths and URLs and at set the following (if they are not already set) -

default_table: Sample
default_tpl: default
default_user_table: Sample_Users

That means that the default table will be Sample (you can set this to any table) and unless you specify another table in the url (e.g http://www.yourdomain.com/cgi-bin/dbman/db.cgi?db=tablename) the default table will be loaded. http://www.yourdomain.com/cgi-bin/dbman/db.cgi? will load the Sample table.
The default template is the part that seems to be confusing you. With DBMan SQL you can set a different template for each of your tables or share the same one for multiple tables. All template sets are stored in the following directory on your server -
path_to_your_server/cgi-bin/dbman/admin/templates. So the above setup is using the default template set which is a set of html files (e.g add_form.html, modify_form.html, etc).
The default user table is the table that stores all the users for one or more different tables. As with the template set, you can have different user tables for different tables.
At this stage set your defaults as indicated above until you understand how it works.
Next, while still in your admin Click on Table. Unless you have already loaded your custom tables you will only see the preloaded tables (e.g Book, Author, Sample, etc) in the Editor drop down menu. Below that you will see the options for creating and loading tables.
Once you have created or loaded your custom tables you can set the templates, usertable, permissions, etc. by selecting the table name in the Editor list and selecting setup in the list below that and then hit Go.
At this stage, your custom tables should work using the default template set and sample users table. You can change them later when you understand how it works.
Hope that helps.
Simon.