This plugin auto-detects Yoast's WordPress SEO, along with 10 other SEO plugins and themes, so you shouldn't have to configure anything.
If you'd like to change setting from default, see line 129 for Yoast-specific settings that can be uncommented or used in your custom code:
'WordPress SEO' => array(
'Title' => '_yoast_wpseo_title',
'Description' => '_yoast_wpseo_metadesc',
'Keywords' => '_yoast_wpseo_metakeywords',
// 'noindex' => '_yoast_wpseo_meta-robots-noindex',
// 'nofollow' => '_yoast_wpseo_meta-robots-nofollow',
// 'Canonical URI' => '_yoast_wpseo_canonical',
// 'Redirect URI' => '_yoast_wpseo_redirect'
I just installed the plugin with WordPress SEO 1.2.4, and verified that it works without setting anything up. You'll see "Title Override" and "Description" columns added to the post listing columns. Clicking the column in the row of your Post allows it to be edited.
If you're trying to add Focus Keyword to the fields, the key is