This worked for me if you don't need all the other stuff Ian built in. Don't know why it's returning lists of lists but I just select the first one.
dt = open("$SAMPLE_DATA/Big Class.jmp");
biv = (dt << Bivariate(Y(:height, :weight), X(:age)));
frame_list = biv << xpath("//FrameBox");
wait(2);
frame_list[1][1]<<Select;
wait(2);
frame_list[2][1]<<Select;