Last yr I used a Windows laptop to prep my tax return using TT. I now have a Mac. I've copied the TT data files from last year to the Mac. How can I access these files?

Ok ... the 2018  .taxfile  can be transferred into the 2019 program.  But if you want to open the 2018 .taxfile you must have the 2018 program installed on your MAC.  

How do I transfer last year's return into the 2019 TurboTax for Mac software?

If you still have the PC then open all the old programs and save the returns as both PDF & .taxfiles to move to the new computer.  You only need to install the prior year programs if you need to amend something.