If you are working in a Blank template and inserting in the mail merge fields such as {{FIRSTNAME}} and {{SIGNATURE}} you have to click on the Manage Templates link to then ‘create’ the email as a template before you will see the MailMerge check box which will then merge in these merge fields into your email. 

The image below shows the merge fields inserted for {{FIRSTNAME}} and {{SIGNATURE}} to see these you must click on the 'Manage Templates' link as shown in red: 

Name your template and then click on Create New

You will then see the MailMerge check box which when checked will pull in your merge fields.