Debugging someone else’s machine and code without being able to see it is hard, but …
The two most obvious reasons are:
(1) you didn’t properly disable HTML text filtering.
(2) you just created a new ad unit. It takes about 20-30 minutes for new ad units to start displaying ads. Just wait.
Checking that HTML filtering:
(1) check that you disabled HTML editting. There are two places this has to be done: (1) at the global configuration page and (2) under the Options -> Text Filters
If these are not disabled, then Joomla! will strip out your HTML code and the adsense code will not work.
(2) Check that the adsense code you added actually looks like what Google provided. Go to Extensions –> Module Manager and find the custom module you added. The easiest way is to sort the ID column in descending order, the custom module you added will have the highest ID.
If the HTML in the Custom Output box is not the same as what Google provided you, then have not correctly disabled HTML filtering.
If you created a new ad unit
If the HTML code is the same as what Google proviced, then you may have to wait about 20 minutes or so for Google to update your Adsense account.
When you create a new Adsense Ad, it takes about 20-30 minutes for it to become active and start serving ads.

You’re welcome and thanks for the feedback.