Here's one way to do it using the summarize command. Summarize yields the list of unique values but they're a list of character strings.
dt = open("$sample_data\bands data.jmp");
summarize(humidity_list = by(:Humidity));
n = nitems(humidity_list);
humidity_mtx = index(1, n);
for (i = 1, i <= n, i++,
humidity_mtx[i] = num(humidity_list[i]);
);
humidity_mtx = sort descending(humidity_mtx);
print(humidity_mtx[1::10]);
[105, 103, 100, 98, 96, 95, 94, 93, 92, 91]