To customize the stock ticker at the bottom of your AIM buddy list, first bring up your AIM Preferences window by clicking the preferences icon in the Buddy List toolbar

or by right clicking your system tray AIM icon.

Now select the "Stock Ticker" category on the left hand side, and you will see the list of Stock Ticker options available on the right.

The options are pretty self explanatory, but to actually customize the stock listings that you will see scrolling, you need to click the "Edit Stocks" button.

These are the default ticker symbols that will be listed.  Use the "Plus" and "Minus" buttons to add and remove ticker symbols.  To add a "Stock Alert", select the appropriate ticker symbol, and then click the "Bell" shaped icon.  This will allow you to set an alert that will notify you if the stock price drops or increases to a specified value.  When you have finished adding your stocks and setting your options, return to your buddy list and you will see the new stock information displayed in the ticker.