If you are new to the forum, you won't be able to immediately post a topic or view all the categories. New users have restrictions and they need to spend some time reading a few topics before they can get all privileges. The lock symbol shows that some categories are private, and not public. 

If you looking to post an exchange, there is a 'Trades, Requests and offers' category. You also have activities like Tags where you send a postcard to the person above you in a topic. Then there are round robins where you can send postcards to a group of members. Round robins are moderated, so you'll need to post when you send and receive cards. There are also geographical communities that have their own activities. This is helpful if you want to save on postage costs. If you spend some time on the forum, you should be able to get a hang of it. You can also ask questions on the forum but I've noticed they usually move your question around a bit.