Giuseppe - the error message is telling you that the code in your ForexFun is NOT using positive integers or logical values when indexing into arrays at the line
if askW(idx)>askW(idx-X(9)) + min(MFL(idx-1),MFL(idx-2))
Presumably idx is an integer and that askW and MFL are arrays (or is one a function?). Since X is the ten element array and you are subtracting the ninth element from idx, what guarantees are there that idx - X(9) is a positive integer? Are you assuming that this is the case or maybe you need to round up to the nearest integer? Or maybe the error is with something else in this line. I recommend that you put a break point at this l ine or add some logging (via fprintf) to write out the indices before this line of code so that you can get an idea of what is happening.
