It looks correct. Make sure you have the right zone id. If its still not working, check the source code of the page and make sure the script url above is included in the head of the html. If not, most likely your template is removing head scripts.
So we've got the openx single page ad call working but it seems for only one zone at a time on a page? Once we include another instance of the module from another zone it will either disable the original module or kill both.