The links for the API samples all point to wikipedia, not wikidata.
The first one (action=wbgetentities) for example, is: https://en.wikipedia.org/w/api.php?action=wbgetentities&sites=enwiki&titles=Marie%20Curie&format=jsonfm So I have to manually change it to wikidata to make it work.
The last one (prop=wikibase) doesn't work whether I change it or not - not sure what the issue is.
I was looking for a way to get the interwiki links for a given wikidata item. It appears that "wbpprop=alias" from this last sample is the solution, but "wbpprop" is an unrecognized parameter.