The "Extended Mail Merge" option is an available option to all Salesforce editions (except for Database.com). If it isn't currently available to your org, you can request activation by contacting Salesforce Support. Here's how:
  1. Before you make your request, review the Salesforce Extended Mail Merge features documentation
  2. Have your system administrator open a case with Salesforce Support
  3. In your case, request activation of the feature and include all important details

After Salesforce Support has activated Extended Mail Merge
Once the feature has been made available by Salesforce Support, you'll need to activate it your org's "Setup":
  1. Click Setup
  2. Select Customize > User Interface
  3. Under "Advanced", select Activate Extended Mail Merge
  4. Click Save