Uploading a Csv of emails to an email list or a message is the same process.
Before doing anything in the event dashboard, make sure your CSV file looks like the one below. The header of the file should be "First Name", "Last Name", "Email"
If your document is in XLS, or XLSX, make sure you File->Save As from the given program and choose "CSV" as the output format/type.
If asked, delimit with a comma.
Once you have the created a CSV in the above format, if adding it to a list navigate to Message Center and then Email Lists. From there find the list you want to add it to and click the pencil icon or add a new list.
Click the Import Items dropdown
Select Upload From CSV
Browse to CSV file
Click Upload File
Click Save Add/Update List to save.
The process is very similar when adding recipients to a message. On the Recipients tab, follow the same steps as above.