An engagement's currency must be set on creation. You cannot take an existing engagement and change the currency. The process for setting the currency isn't obvious, so this help document will help you do it.
Cost Center
Each cost center in your organization is associated with a company. The company will have a currency. New engagement's created will inherit this currency by default.
Client
You may have a specific client that invoices in a certain currency. You can edit that client and choose a rate card in the appropriate currency.
Project Editor
You can override the default currency on engagement creation by going to the project rates tab and choosing an appropriate rate card.
