for example, myDataSet.Tables[0].Rows[0][0]

it gives you a value of the first colum in the first row

Is it a strong or weak typed DataSet?

Let's say you have
EmployeeDS
with one table : Employee
and 3 columns
EmployeeID (int) , LastName (string) , FirstName (string)

Here is some pseudo code:

EmployeeDS ds = new EmployeeDS();

ds.Employee.AddNewEmployeeRow ( 101, "Smith" , "John") ;

........

EmployeeDS.EmployeeRow row = ds.Employee.Rows[0] as EmployeeDS.EmployeeRow;
// friendly cast
if(null!=row)
{
int empid = row.EmployeeID;
}