You REALLY need to upgrade. 0.9.0.12 is so old it's obsolete. I'm pretty sure there was a security release that affected that generation of b2evolution, so unless you've installed it your blog is at risk of hackers defacing it. I strongly suggest you upgrade to 1.8.6 before you put your adsense in, but it's your blog so ...

Open up skins/custom/_main.php and scroll through the code a bit. Eventually it'll be obvious where the sidebar is because there are ample comments and function names are mostly obvious for what they do. Once you find the place you want your adsense code simply stick it in there. I'm pretty sure adsense is javascript stuff, so you want to make sure you're not inside php stuff when you do it. Sorta something like this will be what you should end up with:

			) );
		// -------------------------------- END OF CATEGORIES ----------------------------------
	?>

<div class="bSideItem>
--- your adsense code here ---
</div>

	<?php
		// -------------------------- ARCHIVES INCLUDED HERE -----------------------------
		// Call the Archives plugin: