If you're importing recipients to a Campaign via the CSV method (which you can create in Excel or export from Salesforce), the .csv file will be the source of merged data. You'll need to map columns from your CSV to the email that you're creating in Campaigns.

Create your CSV file. Whatever you name the columns on your CSV will be the names of your merge fields. "Email" must be your first CSV column.

When creating your email from Campaigns, use the column names as your merge fields, using the {!columnname} format. 

NOTE: this is case and space sensitive. If you are using an existing Template in your Campaign, make sure that the columns in your CSV match the {!columnname} in the Template.

To make life even easier, use the "Merge Field" button at the bottom of the compose area to help you match the merge fields exactly with your column headers and quickly insert them.  

Check the "Preview and Personalize" tab to ensure that you've properly mapped your fields and you're ready to send your Campaign!