In Mercury there are multiple places to send an Email from. In this case we will look to Email from a Contact Record
Access the Contact record and click on the Activites drop down. You'll notice different options based on what kind of Contact record you are in. Client, Candidate or Both. Choosing "Email about Candidate" will form an Email with the Candidate CV attached.
Choose the option you want and then complete the fields required by your organization using the Email Editor. and click Send Email when finished.
The Email Activity will show up on the Contacts timeline.