James - just use a local variable to store the turn angle, and update it on each iteration of the for loop. Try something like the following
t=Turtle();
turnAngle = pi/2;
for k=1:5
t.push()
t.turn(turnAngle);
t.up();
t.go(1);
t.down();
t.push();
t.turn(-9*pi/10)
t.go(sin(pi/5)/sin(7*pi/10))
t.pop();
t.turn(9*pi/10);
t.down();
t.go(sin(pi/5)/sin(7*pi/10));
t.pop();
% increment the turn angle
turnAngle = turnAngle + 2*pi/5;
end
