How to install JRE on FreeBSD 8?
You can't (yet).
If you look at http://www.FreeBSDFoundation.org/downloads/java.shtml you'll see that the FreeBSD Foundation has packages for FreeBSD 6 and 7, but not for 8.
There may be another way, or someone else may even have created packages, but if they have, I don't know about it.