Hi luismaya,
My best guess would be to use the Arduino String library. Read the whole line from the file "90,150,200,215,320,420,520". Use the functions you mentioned a few replies back, to search for the "," ASCII code and break the values into strings. Then you can use ToInt() to convert from string to integer, to send the values to your servos. If you are unfamiliar with Strings library and Arduino function, i would suggest taking a look at the following links for reference.
Regards,
Intel_Jesus