The easiest way was to use the Datastore Browser (go to configuration, storage, then right-click on a datastore and select Browse) From there I created an ISO folder and used the browser to upload files to it. These are stored on your VMFS3 partition so it is using up VM space, but I figured we could always delete them later to free up space.
This may not be the “correct” way, but it worked fine for us and didn’t require any other 3rd party utilities.