Sure - the blue navigation bar (blue in the breakroom or off-topic portion of All Nurses) and the yellow navigation bar (at the nursing AN side) are the key.
The navigation bars are in the top portion of the page. Click on one of the topics, say "weight loss" - then you will open up that forum and see the threads. When you have the forum open (with the list of threads) just click on an interesting thread, read thru it and then scroll towards the bottom of the page and you will see a dialogue box - put your response in it, scroll a little bit further and click "post comment".
To start a thread, go to the forum you are interested in and towards the top of the list of threads should be a box that says "start new thread". Click on that and a dialogue box will open up - name your thread, post your comment.
To go back and forth from Breakroom to All Nurses - on the navigation bar (both blue and yellow) in the far right are the labels - just click on that to toggle back and forth.
Hope this helps.