Turn on Multi-Currency first. Then how you add currencies depends on your rate mode.
With automatic rates
With automatic rates, the Additional Currencies list shows every supported currency with its current market rate. Just turn on the ones you want to sell in and choose a rounding rule. You can select several and enable or disable them together.
With manual rates
With manual rates, Add Currency lets you pick one currency and set its rate, with the market rate shown as a reference. Bulk Add lets you add several at once using market rates or one rate you type. You can add up to your store's currency limit; the buttons disable once you've reached it.
Disabling and removing
Disabling a currency hides it from shoppers but keeps its settings. Removing deletes it from the store. If a market is using the currency, take it out of that market first.