Google Affiliate Network (GAN) is a different type of advertising programme: instead of getting paid per click, you get paid per purchase (or whatever other key action the advertiser decides).
So far, it's limited to Blogger users who are based in the USA - presumably because this is where most advertisers are. It's also limited to "select bloggers" - this seems to mean ones whose blog is a good fit with the available advertisers, and the criteria for "a good fit" is as decided by Google. This is unlike regular use of the Google Advertisers network, where you apply to a particular advertiser based on your own assessment of the fit.
After you have signed up for AdSense (and, I guess been approved!):
IF GAN options are available to you
THEN they will be shown when you are editing a post, in a gadget that appears in the right-hand Post Settings area.
This means they will only available from the "new", ie post-Sept-2011 blogger interface.
I guess you will be allowed to copy the HTML code for the GAN advertisement from the post editor, and put it into a regular gadget - since you can put regular GAN adverts into your blog in this way. But this is a detail that should be checked against their terms and conditions: as with AdSense, the additional requirements for GAN are glossed over when you are accessing it via Blogger, but I expect that they will still apply.
