When adding a deal, you can chose the currency from predefined currencies. You can also change deal currency when editing a deal.
Every Centrium user can have his own default currency in which he adds deals. By default its set to the account owner default currency setting. Changing the default currency does not override current deals currency.
Changing user default currency:
To change user currency, click on your name in the top right corner, go to "Profile and settings", and select the "Regional settings" tab
Changing system(account owner) default currency:
Its exactly the same, just log in with your account owner account and change the regional settings.