Once you have entered your stocks in your profile, click on one of them and you will see this:
To the right, you will notice a listing of the most current tweets on the stock from your portfolio. Below, you can find links to view that stock on Google Finance, Yahoo Finance and others, along with Twitter Links - stories from other online sources.
When you want to write something about your stocks or other stocks, write it in, include "$" followed by the ticker symbol and post it (see below.) It seems that the post will not go on the site unless you include the "$SYMBOL" in the post. Your post will then show up on the StockTwits site, as well as your regular Twitter account.