There are several ways to do such a mail merge. What method did you use? Some methods would make this very easy, with others it will be impossible.
When in the database that I've created I clicked on New Layout/Report then followed the instructions that come with filemaker.
In what version of FileMaker?
What options did you select in the New layout/Report Wizard? (There's no option for mail merge that I can spot in the first dialog to pop up in FileMaker 12 when you select New Layout.)
The key detail is that if you created a layout that looks like this:
Thank you for purchasing the <<ProductNameField>> system.
Then your mail merge combines both layout text and data from fields to produce the mail merge. The layout text cannot be edited while in browse mode and any edits to that layout text would affect all records you then printed or saved as PDF.
There are other methods such as a setup where the message template text is copied into a text field and a calcualtion field uses the substitute function to replace "placeholders" in the template text with data from the current record. Since the template text is now in a field for each recipient of your message, you now can edit this template in order to further update it to customize it to the needs of each recipient of your message.
If you are sending out emails, A layout based mail merge won't work at all, but the second method that I am outlining here can be used with the Send Mail tool to send out plain text emails as well.