The currency list is set in the language file General.pm, the variable currencies_list. Edit this list as you like to include the currencies you want to be displayed on your site for users to select and it should include your site default currency.
The list is formated in the form:
Currency_ID:Currency Name|Currency_ID:Currency Name| ... |Currency_ID:Currency Name
The Currency_ID is the Unique Internationl Currency Symbol and The Currency Name is whatever you name it in your language or other languages. If you have more than one language installed, keep the Currency_ID the same for all languages and change the currency name for each language.
USD:USD|SAR:Saudi Arabian Riyal
Now this list will display in your admin center:
admin -> General Options -> Default Currency:
Form this option you can set your default currency.