There are numerous ways of doing this...
A. Custom Islands can be adapted to hold PHP Code, Markup (HTML), or Text; so you could insert code there and set them to be one of the sidebar blocks OR a 2nd post island (like at UBBDev for example).
B. You can hardcode them into the template files as pure HTML; not really recommended as it's not scaleable, but if you want the ad's in a place other than mentioned above it's the only option.
