You can open them with InDesign CS6 or earlier with a couple of caveats.
They must PM 6.0 or later.
Results will be anything from near perfect to worthless
All images much be linked externally.
InDesign CC and CC2014 will not open PM files. CS6 is included with your CC subscription.
In CS6 or earlier File > Open should open PM 6 or 7 files.