Hi Anne -- happy to help, don't ever hesitate if you have questions, as you say the answers will probably help others, too! :)
The page you're trying to click on is the 'register' page, but you'll get that 'access denied' message if you're already registered. To get into the network from the website homepage, just click on the green button at the top right of the page that is labelled 'network home'. Then you should see your own pink profile box a little bit down the page on the right-hand side (if you're not logged in, it will give you some login fields). I agree the 'access denied' message isn't very helpful as a message, we have it in our queue of things for the developers to fix.
For information about how the site works, the best place to start is the site's Help page and Q&A page (there's a link to these up the top of every network page, just under the main navigation on the right of the page). The Help page will also give you the link to the All Posts page as well, if you lose it (there's another link on the network homepage, in the main panel) .
This site is a little bit closer to Facebook, than to topic-based forums, because our aim was primarily to help people connect with each other first, rather than have a topic-based forum. So if you go to your own 'home' page, you'll see a list of recent posts of people you've connected to there (a bit like your facebook newsfeed, if you're familiar with that).
To add a tag -- when you make a new blog post, you'll see a field right up the top labelled 'tags' right under the Title field. It has a little bit of help text under it just letting you know to use commas to separate each one, then they'll all be clickable. We're thinking of having a 'tags' week soon, to get everyone going with tagging posts, because it's a really useful way to find topics of interest.
To find all the tags other people have used, go to the All Posts page, and you'll see a panel on the right side under your pink profile box labelled 'Filter by tag'. If you click on the little 'more' link, it will take you to this page, where you can see all of the tags.
Hope that helps, Anne -- just ask again if anything doesn't make sense.
~Di
