I want to be able to buy a licence to use adode in Nicaragua? can I buy the licence using an American credit card or what are the steps to follow.
I am also confused on why is it not for sale in certain countries.
As long as you purchase FormCentral using a payment method with billing address in a supported country you can use that membership from anywhere. Purchasing with a Americanpayment method (billing address) and using in Nicaragua will be fine.
I'd make sure that the language selected for the site while you are setting up your account and/or purchasing is set to English/US. After you've set up the account you can change the language of the application and use it wherever you are.
This FAQ might help in case you run into issues setting it up: http://forums.adobe.com/docs/DOC-2984