Of course it is C[i] = A[i] + B[i].It was a typing mistake,never mind. You just understand the concept.
for k=0 to n-1
This will make n sub branches all to be executed in parallel and finally sync instruction will cause the flow of execution to be synchronized at that point.
x1 is the constant to represent computational cost of spawning a new process. This step is executed (n/grain-size) number of times. x2 is the constant to represent the computational cost of the addition operation. This step is executed (grain-size) number of times for each spawned preocess in parallel. Hence total time for a parallel thread is x1(n/grain-size) + x2.(grainsize)