Each account has one currency; change it by editing the account. Accounts in different currencies are part of the Gold plan.
The primary account (the one with the star on the Accounts page, or in Settings › General › Primary currency) sets the primary currency: every amount is converted to it in the totals.
Convert while entering an amount
If the account chosen for a transaction is in another currency, the conversion button, when you type the amount, turns the amount into the primary currency and moves the transaction to the primary account.
Which rate counts
Each transaction in another currency keeps the BNA rate of its own day: its value in the primary currency doesn't change later. Balances and net worth use today's rate.
Exchange rates
Macuta uses the reference rates of the National Bank of Angola (through Finda) for every currency the BNA publishes; the others come from a public source updated every day. In Settings › More options › Exchange rates you can set your own rate for any currency.