By enabling the Multicurrency setting in Setup, a field with currency options is displayed in the vendor setup window so a different currency can be selected.
Instructions
The vendor's window should look like this before you start to setup the exchange rate:
To complete accordingly the setup, please follow the next steps:
Go to Setup > Settings
In Company Setup section, click on Currency Options link.
In the pop-up window, confirm the Home Currency is displaying by default the current currency.
Confirm Multicurrency checkbox is unchecked.
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: