Myxye on "[Plugin: WordPress SEO by Yoast] Auto meta description isn't working"

WordPress version: 3.5.2
WordPress SEO version: 1.4.12

I did this:
I have setup %%excerpt%% in the Meta Description field for Articles.

I expected the plugin to do this:
Auto complete meta description.

Instead it did this:
I don't get auto keywords.

http://wordpress.org/extend/plugins/wordpress-seo/