Hello Minimalist,
I suspect that dynamiharry may have put the code in the child theme's "functions.php" file. I hope you're using a child theme for your website (more info on it here http://themeshaper.com/thematic/guide/?page_id=66) if not why you should (if you are already using a child theme, just skip that link) :)
If you don't yet have a child them "functions.php" file you can create a new one, just name it functions.php (or see the Thematic sample child theme that comes with pre made one in the download of thematic). An Open PHP tag will look like this:
<?php
add the following code after it and it should work fine:
//Function that creates the opening DIV tag above the Main Asides
function webmagic_divabove_sideopen() {
echo ('<div id="yournewdivname">')
;
}
add_action ('thematic_abovemainasides' , 'webmagic_divabove_sideopen');
//Function that creates the closing DIV tag below the Main Asides
function webmagic_divbelow_sideclose() {
echo ('</div><!-- This is the end of Your New Div -->')
;
}
add_action ('thematic_belowmainasides' , 'webmagic_divbelow_sideclose');
There are plenty of people more experienced than me on here who are brilliant and may have other suggestions but I tested the code above and it worked fine! Note, what the above is doing in simple terms is Thematic has been cleverly designed like a wall with velcro hooks on it in key places in the website. The code says take what I put in my echo statement and attach it to one of Thematic's velcro hooks, in this case we are targeting the hooks that are helpfully called thematic_abovemainasides or thematic_belowmainasides.
Others on here may have other thoughts or advice but that's my take on it!