You can't reply to support threads until you've got over 500 or so posts (can't remember the exact number). The ability to reply to threads other than your own in the support forums are restricted to a higher user group to stop thread hijacking.
I also see that this is your first post. Until you get about 5 posts made, you'll be required to fill in captcha and have other restrictions. This is also in place to stop the spamming of the forums by bots. I've learned that the posts in the introduction forum don't count in your post count, too.