If you see a blank screen and unable to load the MassMailer Template Editor under the MassMailer Template Builder Tab, it must be because of the session settings in Salesforce.
Navigate to Salesforce Setup -> Quick Find 'Session Settings' -> Go to the section 'Clickjack Protection'
Uncheck the following options which should fix the issues with the editor.
- Enable clickjack protection for customer Visualforce pages with standard headers
- Enable clickjack protection for customer Visualforce pages with headers disabled