The Extension's Configuration is in the admin menu: System > Configuration > Sales > Payment Methods > Walkin. This is a setting for creating a shipment while the order is being created, the order status can also be specified. This payment method only works in the admin.
For all other offline payment options, navigate to System > Configuration > Sales > Payment Methods:
Purchase Order Option:
Cash on Delivery Option:
Bank Transfer Payment Option:
Check Money Order Option:
During the checkout process, your customers will see all the payment methods you have enabled: