probably the easiest way to do this is to enter the value of the key with a delimeter (eg a comma). Use the string's Split function to split the values and there you have your array. Depending on the type of your needed array, you'll need to convert.
config file
<add key="myarray" value="1,5,6,8,9" />
Code behind
string myarrayvalue = ConfigurationManager.AppSettings["myarray"];
string [] str_array = myarrayvalue.Split(new string[]{","}, StringSplitOptions.RemoveEmptyValues);
int [] myintarray = new int[str_array.Length];
for(int i = 0; i < str_array.Length; i++){
int output;
if(int.TryParse(str_array[i], out output){
myintarray[i] = output;
}
else{
myintarray[i] = -1;
}
}
This is a pretty basic question, please start out with a good book.