It's hard to describe with words. I would suggest you to watch video reviews on how you can attach your car seat. Also you can get help from your manual book which you got while purchasing the stroller. Hope that will help. Visit here for more: Best Twin Pram For The Money in 2017 Tested and Reviewed
The park brake cable looks very similar to a bicycles brake system.
Buy a spare bicycle inner cable of suitable length and then cut to size.
Or else take your pram to a cycle shop and a smile will probably get this fixed...
Bregds