I needed to put an adsense unit into the category page of the wordress theme 'twenty ten' and found an easy way to it. I am using wordpress version 3.3. This is what I did.
1. Open notepad and copy your adsense code into it.
2. From yor wordpress dashboard select the Editor hyperlink from the Appearance menu.
3. On the right hand side of the page that appears, click on the Category template (category.php)
4. When the file is opened for editing find the line <div id="container">
5. Directly after this line copy and paste your adsense code from notepad. This will put an ad at the top of the page.
6. Scroll down the page and find the line which says </div><!-- #content -->
7. Directly after this line copy and paste your adsense code from notepad. This will put an ad at the bottom of the page.
8. Click the Update File and voila two google ads on the Categories page!
Take note though that if you are using a plugin similar the Advertisement Manager and have configured it to serve ads at certain places on the page that these will be displayed also and you could go over the maximum of three ad units allowed on a page. Other than that. Easy.
You can also do the same trick for displaying ads on the Authors page but editing the Author template (author.php) in the same manner as described above.
To see the result then you can check out my category and author example pages at
http://www.4youcoupons.com/category/baby/
and
http://www.4youcoupons.com/author/admin/
Good luck.