Each Campaign can have it's own default currency that will automatically carry in from the Advertiser. You can override the currency for the campaign in the Basic Info page. The ability to allow for Multiple Currencies will also be carried over from the Advertiser settings, and can be turned off here.
If this campaign should support multiple currencies, you will want to turn multiple currencies on by clicking on the icon.
Disabling multiple currencies will make the Currency field in the media plan inactive.
Next Step: Campaign Settings – Production Costs