As the Administrator, you can activate this feature by adding the home currency.
On activation:
To activate multiple currencies and add a home currency :
Click Setup > General > Company Details
In the Company Details page, go to Currencies tab
Select the required Home Currency from the drop-down list.
Note that the home currency cannot be changed. The format of the selected currency will be shown. 
Click the Customize link to change the following:
Select the Thousand Separator from the drop-down list.
Select the Decimal Places from the drop-down list.
Select the Decimal Separator from the drop-down list.
Click OK and then click Confirm.