Conga Mail Merge is a free add-on to Conga Composer and requires separate installation. Follow the below steps to install.
Click the appropriate link (Production or Sandbox) to install Conga Mail Merge in your Salesforce environment from the Installation Links for Conga Products page.
Log into Salesforce using the appropriate username and password for that environment and follow the on-screen prompts. For Security Level, we recommend “Grant access to all users.”
After installation, you may access Conga Mail Merge from the Force.com app menu.