In where it says define file to run you have to add the following:
/whereyouinstalledsmf/Sources/shop/dointerest.php > /whereyouinstalledsmf/interest_log.htm
replace whereyouinstalledsmf with the directory where you installed smf if you did install it in a different directory, you can go see in admin, server settings, SMF Directory.
Tick execute file at this time:
and select from Days of the week if you want to calculate interest on a certain day of the week, else you can do it on a specific day of the month you like, or even worse do it once a year by selecting a Month.
Then I think you just need to click on schedule to run schedule it.
You should be able to see the interest log by typing your url/whereyouinstalledsmf/interest_log.htm
