Where are the popcorn time temporary files stored on OS X Mavericks?
I found the answer here.

Open Terminal.app and enter the following command: 

open $TMPDIR/Popcorn-Time


This will open the Popcorn-Time storage directory, and from there you can copy the file away to where you want it.

Edit:

Now

open $TMPDIR/Butter


Edit:

Changed again,

open $TMPDIR/Popcorn\ Time