By enabling this setting, the home currency can be changed in the main screen and then, from the Setup screen.
Instructions
To complete accordingly the setup, please follow the next steps:
Go to Setup > Settings
In Company Setup section, click on Currency Options link.
A pop-up window will display. There you should validate the home Currency that is displayed by default. The Multicurrency checkbox will appear as uncheck.
Then, check the Multicurrency checkbox and confirm the following information is displayed.
Check the "I understand I can't undo Multicurrency" option and click on Save button.
Click on Currency Options link again and confirm the window is displayed as follows: