on my phone, I put the URL into my Phone's Browser, and a stripped-down version of the Forum pops up.
I have to scroll all the way down to the Bottom, and there is a "Login" button. hit that, log in, and then you should be able to post. just click on the appropriate section, and thread, and then scroll all the way down to the Bottom and there should be a "reply" button, or if you are making a new post, open the thread list, and scroll to the bottom and there should be a "New Topic" button.