Before you can actually download JBDS, you have to register. Just select Register for Free Here.
Select Sign Up and then select Create a personal Red Hat login. At this point you may be asked to confirm your e-mail address. And finally, you'll see the download page, as shown:
Select the download that matches your computer system and architecture and save the download file. The file name will look something like jbdevstudio-product-linux-gtk-x86_64-4.1.0.v201108011413R-H647-GA.jar.