What X11 program are you trying to copy from/paste into? Are you using Apple's quartz-wm or a standard window manager? Apple's quartz-wm supports copy and paste between Aqua and X11, other window managers do not.
AFAIK, autocutsel merges clipboards within X11 (the "normal" clipboard and the left/middle-click clipboard), not between X11 and external sources (ie OS X pasteboard.)
Also bear in mind that any X11 application in which middle-click is used for paste will only paste the the currently selected in X11, which in turn will clear the selection, making copying and pasting via this method somewhat counter-intuitive and a one-shot deal.