To check it out go to Sales > click the down arrow next to Export:
Then click Preferences and you'll see this overlay:
Just click the Endicia option and every export will have another file specifically for that import. Then in the DYMO Stamps program, which is the Endicia for Mac program, you would click the Import Address File on the bottom right:
You'll get this upload box:
We include a header row, so keep that checked. Then choose if you want to add it to your current book, or write over it. This all depends on if you finished shipping all the ones you imported.