do you have your own domain or is it hosted by them. If hosted by them, you can't. If it's yours your best bet is to get an adsense optimize template to use.
It is impossible to add Ads to Wordress hosting unless you have your hosting. But you can do it in blogspot.com It is free totally.
You do it with a widget or by editing your template. This of course does not apply to wordpress.com sites only those you host yourself.
If you are hosting the blog then you'll have no problem adding adsense. You simply go to the sidebar and add a text widget, in the text widget add the adsense code. It should now display in your sidebar.
You can use different adsense plugins to put ads in your blog, however, I personally prefer to put ads in themes manually.
Yeah! manual ads works fine for me! but newbie cannot ad them easily! it needs template editing to be done