The currency displayed on your project is determined by your project’s country of origin. That means if your banking and identity information is based in, say, Canada, your project automatically will be in CAD, if you’re in the UK it will be in GBP, and so on. At this time, there is no way to choose a preferred currency.
Backers based in the US will also see each project's goal displayed in USD (even if the project is based outside of the US):
They'll also see reward tiers listed in USD, with the project's native currency alongside: