That's not exactly the code from the thread you linked to. I suggest to take that code (the little snippet that i enhanced with comments should be enough) instead and see if it works. If your plane is aligned to the xz-plane, you can calculate the intersection point much cheaper...i've done this in a prototype game for Android, but i can't access the sources right now. I can post it next week if needed. However, the approach from the mentioned thread should work fine too.