for some reason youtube doesn’t currently have a link from the normal landing page for a playlist to any way of actually playing the selection of videos end-to-end.
this could be because: they are trying to do something clever around blocking third parties or in preparation for google tv or in terms of advertising; or because they are disorganised and have broken this core feature of a playlist; or because the button is there, but I can’t see it because of bad UI.
anyway, you can fix it with this firefox keyword bookmark:
view play list
of course, as with all things youtube related, this fix will be redundant any time soon when google switches round all the functionality and code again.