How can I add some code to evrypage appear at the bottem?
where abouts exactly do you want it? and what code?
^^ what code? static html/js or dynamic output?
in general, it's code is added to header template ( near logo ) ..
however, many prefer adding such social network buttons (eg. twitter , fb like, digg ...) on showthread template ( at the top )
if you want it on all pages, just copy the code they give into the header or footer templates. Pretty simple to do.