Scheme:

(define (pickNumToAdd M N)
  (if (null? M) 0
    (+ (nth N (car M)) (pickNumToAdd (cdr M) N))))


Prolog:

pickNumToAdd(M,N,Out) :-
  M==[] ->
     Out = 0
    ;
   (
     [CAR|CDR] = M, nth(N,CAR,NTH),
       pickNumToAdd(CDR,N,MID),
          Out is NTH + MID
   ).


Frontal:

f_nth(A,[A|B],1).
f_nth(ListItemValue,[A|B],N) :- N>0, N1 is N-1, f_nth(ListItemValue,B,N1).

f_car(CAR,[CAR|_]).
f_cdr(CDR,[_|CDR]).

f_pickNumToAdd(Out,M,N) :-
  M==[] ->
     Out = 0
    ;
   (
     f_car(CAR,M), f_nth(NTH, N, CAR),
       f_cdr(CDR,M),
       f_pickNumToAdd(MID,CDR,N),
          Out is NTH + MID
   ).