Facebook "like" button on first post!

[ROTPAR]

Can anybody please explain to me how this is working for the facebook "like" button? For example: How do I get this code to work in the first post of each thread?

PHP Code:

<vb:if condition="$show['fb_likebutton']">
            {vb:raw fblikebutton}
        </vb:if> 


I placed it together with my twitter button in the ad_showthread_firstpost_start template. the twitter button appears but the facebook button doesn`t. Somebody told me I have to register the variable? I have no idea what this means.