Go to Themes > [Active Theme].
Now on the left-hand menu click Left Column, Right Column, or Page Header. This will bring up the editor for that area.
You will see a list of snap-ins you can add and all the ones currently on the column. Edit them accordingly.
If you want raw html content, just add a "Custom HTML" snap-in and a WYSIWYG editor will appear. You can switch between the WYSIWYG and text HTML editors.
From here you can add multiple snap-ins and change their order, etc.
For adding an AmeriCommerce managed list of links add a "Link Group" snapin. You can manage these links from the Site Explorer.
For adding a list of categories right from your AmeriCommerce product catalog, add the "Browse By Category" or "Browse By Manufacturer" snap-ins.