If you are using Microsoft standard firewall then it is in the control panel / system & security section / firewall.
You can add popcorntime as a 'trusted' or allowed application. If you are using any other firewall then there will be a section to add trusted files or whitelist programs.
On a separate note if you are using Microsoft's then you should consider upgrading.
Try http://www.comodo.com/home/internet-sec ... ivirus.php
Its the best free AV out there
ps. I can also send you steps on adding popcorn time as a trusted file for Comodo.