ISA Browns are a crossbreed, and as such, do not breed true. If you want to know how to breed your own sexlinks, check out this thread, http://www.backyardchickens.com/t/261208/sex-linked-information
All the good info is in the first post.
If you're wanting an adult rooster, then wait till the hens are laying well before adding him. If you want a young male, than get one that's about 10 to 12 weeks old. Introduce him when the hens are all mature and laying. He will be young enough, that a few pecking order scuffles with the older girls will keep him well behaved for a good, long time.