I don't know that particular lens, but it looks as though it might be one of those where the whole front optics assembly simply unscrews from the barrel. Check first for any little grub-screws that might be retaining it, then hold on to the barrel firmly and try to unscrew the front section.
Is that ring you have removed the retaining ring for the front element? If it is, and you want to remove the element, place a soft cloth on the bench and bring down the lens sharply and squarely to free it. If you don't need to take it out, put that ring back smartish, before the element decides to drop out of its own accord and land on something hard!