Using Mega Sign and merging data fields

I've spent hours trying to figure out how to merge custom fields from the csv file into the mega sign process.  Why must it be so difficult? Office does this so easily.  I have more than 1 email in the file. I even tried creating a pdf form with the exact same custom field names - even tags.  No luck. 

We have 80 letters to go out with custom data for each one but the template is the exact same for every letter. It shouldn't be this difficult...... 

Any advice?

Thanks. 

Greg.