it depend on where you want to put that code you can put in layout file if you want to put it on product page or somewhere else.
The easiest is using CMS blocks, this is a very simple approach however there are other ways.
If you create the following file
app/design/frontend/default/yourwebsite/layout/local.xml and add the code below, in admin you then need to set Configuration > General > Design > Themes > Default
to yourwebsite or whatever you name the your folder as above.
<?xml version="1.0"?><layout version="0.1.0"> <default> <reference name="right"> <block type="cms/block" name="right.static"> <action method="setBlockId"><block_id>right_static</block_id></action> </block> </reference> </default></layout>
Create a new CMS static block called right_static and paste your adsense code, clear your cache and you should see the ads immediately. Adsense initially places blank images if it is a new account so you should check the source html in the browser if it is not showing.