In Settings -> Card Options you can integrate your Tally account with a payment processor for taking card payments.
If you chose to use Worldpay then you will need your "Installation ID".
To get your Worldpay Installation ID you need to email firstname.lastname@example.org.
Tell them that you are using KashFlow and give your Tally username. Say that you need 'Invisible XML' enabled and that you need an Installation ID.
Once you have it you can enter it into the appropriate box on the page.