You have 4 options here: You can add recipients one by one. You can import a spreadsheet or upload a CSV file. You can also use a Group in your Google Contacts list.
When you have loaded your recipients click on NEXT, and open the page where you compose your email.
Tip: If you are having trouble uploading your .csv file to Mail Merge we suggest you drop your file into your Drive then import this Spreadsheet to Mail Merge.
Don't forget to check the option Convert uploaded files to Google Docs editor format" from the Drive settings menu.