You don't need the CSS upgrade to add HTML to your sidebar. What are you trying to add? Paste it here between backticks, those little marks on the same key as the ~, so it's backtick, code, backtick. That makes it display so we can take a look at it.
The CSS upgrade allows you to change the look of your blog by editing design elements like colour and column width and height. There is virtually no support for the CSS upgrade, so you are advised not to purchase it until you're good enough to use the free preview to get what you want.
What, exactly, are you trying to change on your blog?