Per the manual:
MLM requires specific labels, especially spine/pocket labels. We suggest you experiment with our sample data base and a plain sheet of 8 1/2 X 11 paper before committing to any one design. This way you will have the opportunity to see exactly how the final product will work. The are some labels that appear to be the same but have a smaller or larger font or may or may not have the barcode on the spine label.
Sources for mailing labels and barcode labels:
Avery 8160 - white copier labels, 30 labels per sheet
Any 8 1/2 by 11 inch paper with 30 (3 columns X 10 rows) labels per sheet will work correctly.