Just a small logical change is needed to solve this problem.

Eg:
******************
for(i=1;i<=n;i++)
{
ddt_set_row(t,i);
set_window ("Inputs", 2);
a=ddt_val(t,"I1");
b=ddt_val(t,"I2");
edit_set("ThunderTextBox",a);
edit_set("ThunderTextBox_1",b);
button_press ("Ok");
edit_check_info("ThunderTextBox_2","value",a+b);
}
******************
This 'for' loop takes values from 2 columns of Excel sheet (I1 and I2) and adds them in the application after clicking 'OK' button. Then it checks the application output (Actual, 'value') with Expected (a+b).

Now, here if you want WinRunner to get the data only from odd rows of the Excel sheet then just change counter increment part of 'for' statement. It can be changed as below:

for(i=1;i<=n;i=i+2)

This 'for' loop is executed with values of 'i' in the following series
1,3,5,7,9..........................

This is one way to get what you want. There are many more ways.