If you don't see Manage Currencies as an option under Setup -> Company Profile then either you don't have multiple currencies enabled (contact Salesforce to enable this) or you don't have "View Setup and Configuration" and/or "Customize Application" enabled in your profile.
Hi,
The currency management is a feature that is not by default enabled by Salesforce.com, although it is free of cost.
You have to contact Salesforce.com to activate this feature.
Thanks and Regards,
Dinesh Nasipuri
Dinesh.Nasipuri@gmail.com