Go to "System" > "Magento Connect" > "Magento Connect Manager". Go to "Direct package file upload" section, pick the module you have just downloaded and click on the upload button.
Let's try the module. Go to "System" > "Configuration". Now on the left menu should appear a new option called "WorldCoo". Click and place there the sandbox credentials we just emailed to you.
By default, WorldCoo's widget appears on the checkout's payment section, you can change that by clicking on "Show advanced settings" on the configuration panel. Once there you can modify the option called "Widget display". Depending on which modifications you want to apply, they can require programming.
Go over your checkout and make a donation test. The widget should appear on your payment method step (by default).
Only payed orders generate valid donations, in order to get an overview over the situation, you can go to "Sales" > "Donations history", there you can check which orders have generated a donation. Over each order you'll see the WorldCoo's donation ID, you can use that identifier to reach WorldCoo over any question you may have.
Once tested, you can install the same module on production. Go to "System" > "WorldCoo" > "Advanced settings", put your production credentials and change "Production environment" to "Yes"