google_alternate_ad_url
 

http://www.google.com/support/adsense/bin/static.py?page=alternateads.html#html

1. you must specify the URL of the ad that you want to use as your alternate for the Adsense.

for example if your adpage is http://mydomain.com/ad.html then on the code it should be written


....
google_alternate_ad_url = "http://mydomain.com/ad.html";
...

And if you are using a script it should be written:

....
google_alternate_ad_url = "http://mydomain.com/myadscript?someotheroptions1234";
...

And it is true that the alternate ad is placed in an iframe that Google Adsense automatically created.

So in your case, you are using this code:
the URL that you use is not actually an ad page but a base domain of a website. You must change that with the URL of an ad page or ad script.


2. Just make sure it's the same size as your ad unit. E.g if you use 468x60 then you need to create a 468x60 banner.

Then all you need to do is create a html file for example. myad.html and insert the following code into the file:
And you should be all set.