how can I read the serial port from arduino when I enter the chars and 'enter' in the keyboard
First, 'Serial.println("loop")' is going to execute every time loop() is called, whether there are characters available or not.  That would show you that the loop is executing but it's likely to obscure the output when you get any.

Second, the while() loop isn't going to wait for you to finish typing; it will only collect the character (or maybe, two) you just typed and then exit, printing out the partial sentence each time.

Third, '*line += (char)Serial.read()' doesn't does something quite different from what I think you intended.  If you're trying to put characters into an array, you need either an index or a pointer into the array which you increment after storing each character.

Here's an example (there are other ways to do it to avoid where some duplicate code, but this way is closer to your original program structure):

char line[80];
int count = 0;

void setup() {                
   Serial.begin(9600);
   Serial.println("loop");  // prompt for a sentence
}

void loop() {

   // loop() will called faster than you can type so there's
   // no need for the next block to loop; you need only test
   // once for the presence of one or more characters.
   if (Serial.available() &gt; 0) {

      line[count] = (char)Serial.read(); // store the char
      if (line[count++] == '\r'){        // if its a CR,
         line[count] = '\0';             //  zero-terminate it
         Serial.println(line);           //  and print it.

         Serial.println("loop");         // re-prompt
         count = 0;                      //  and reset the index.
      }
   }
}