Olvidas el nombre del fichero en la declaracion de tu función.
Código vb:
Ver originalFunction HallarLetra(byval nLinea as long, byval Letra as long, byval Fichero as string) as string
dim numfichero as integer
dim linea as string
dim f as long
on local error goto errorsub
numfichero=freefile
open fichero for input as #numfichero
for f=1 to nlinea
line input #1,linea
next f
hallarletra=mid$(linea,letra,1)
errorsub:
if err.number<>0 then msgbox err.description
close #numfichero
End Function