Navigate to Settings > Configure system > Toggle features.
Click on the Features tab and toggle on Multicurrency. This will enable a link to edit multicurrency settings via the Multicurrency Toggle itself. Click on this link.
Tick the currencies to be used. These will appear in the currency drop down box when creating invoices, for example.
To set a manual exchange rate, enter the chosen value into the Set Rate box. For variable daily rates taken from the XE.com website, leave the rate box empty.
Scroll to the bottom and hit the Save button to update the system.