What you want to do is to show the message if the string doesn't have any letters OR it doesn't have any numbers.
You don't need a regular expression for that either:
if(!str.Any(Char.IsLetter) || !str.Any(Char.IsDigit)) {
MessageBox.Show("Please Enter both numbers and letters");
return;
}
Note: Chech the specification for Char.IsLetter so that the set of characters is acceptable for what you want to do.