It seems documents related to publisher APIs have not been updated yet. With API Manager 1.6.0 there has been a change in the way endpoints are stored. To make the REST API call working can you try with a request similar to following;
curl -X POST -b cookies http://localhost:9763/publisher/site/blocks/item-add/ajax/add.jag -d"action=addAPI&name=YouTube1&context=%2Fyoutube1&version=1.0.0&tier=Gold&responseCache=disabled&visibility=public&endpoint_config=%7B%22production_endpoints%22%3A%7B%22url%22%3A%22http%3A%2F%2Fgdata.youtube.com%2Ffeeds%2Fapi%2Fstandardfeeds%2Fmost_popular%22%2C%22config%22%3Anull%7D%2C%22endpoint_type%22%3A%22http%22%7D&endpointType=nonsecured&subscriptions=current_tenant&resourceTemplate=%2F*&resourceCount=0&resourceMethod-0=POST&resourceMethodAuthType-0=Application&uriTemplate-0=%2F*&resourceMethodThrottlingTier-0=Unlimited&tiersCollection=Gold"
When decoding the request you can see that endpoint is now defined as a JSON
endpoint_config={"production_endpoints":{"url":"http://gdata.youtube.com/feeds/api/standardfeeds/most_popular","config":null},"endpoint_type":"http"}