lifeofthedifferenlyabled on "[Plugin: WordPress SEO by Yoast] Meta description"

so I am trying to switch from all in one seo to this and the meta descriptions i enter in the box under the page or post don't show up in the google results. it shows what looks to the be the beginning few sentence of the post how do i fix this also how do i have it show up as a default description if nothing is entered in the field?
I am using the theme weaver 2 please help!
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http://wordpress.org/extend/plugins/wordpress-seo/