To have default client letter in print, you need to change two file names inside mytaxexpress installation.
For example of myTaxExpress 2013 software,
1. In folder c:\mytaxexpress-efile\mytaxexpress\2013\pdf
change file name from "t1_clientletter_example.pdf" to "t1_clientletter.pdf"
2. In folder c:\mytaxexpress-efile\mytaxexpress\2013\xml
change file name from "t1_clientletter_example.xml" to "t1_clientletter.xml"
then print the tax return again, it will have default client letter printed out.