[Plugin: WordPress SEO by Yoast] Where is my excerpt?

Before I begin explaining my problem: Keep up the good work!

The problem:

In the “Titles” section I’ve set the following for my posts:

%%title%%
%%excerpt%%

The problem is that there is no excerpt visible in the source. It tells me the following (in the source):

Admin only notice: this page doesn’t show a meta description because it doesn’t have one, either write it for this page specifically or go into the SEO -> Titles menu and set up a template.

I know it’s empty… That’s why I configured the titles. Any idea on why this doesn’t work?

Hoping for help!