An average is simple to calculate, there's no need for Linq.
Your code:
0) you never terminate the 'for loop (posting error ?).
Keep it simple:
0) declare a type Double running-total variable outside the 'for loop: update it with each new user-entered value.
1) fill the array with user supplied values: decide if the user must enter a value for each element of the array-size you define, or if the user can quit at any time.
1a) use Double.TryParse [
^] to get valid floating-point values.
2) when you exit the 'for loop:
double average = runningtotal / arr1.Length;