"End of post" plugins not showing up in theme

I use a custom theme for my WP, fairly basic code. I have a couple of “end of post/page” plugins (nRelate, Shareaholic buttons) that used to work in other themes but not in this one – they don’t show up. I am not able to find out how the plugin “knows” where to stick the plugins in, could you help me?

(I’m very much a beginner.)