How about now?
Closer, but still no cigar.
You have no bounds checking on the value of pos. Its value should stay between 0 and 180.
if (digitalRead(buttonleft) == HIGH);
if (digitalRead(buttonright) == HIGH)
Spot the difference.
Have you got any pulldown resistors on the button pins to stop their voltage floating and providing spurious inputs ?
As written, holding down a button will rapidly cause the servo to move one way or the other (once the code is fixed, of course). Is that what you want or should the servo only move a little each time a button becomes pressed and not move any further until it is pressed again ?