The install agent insisted on firing up Safari for me. There was nothing I could do with the .jnlp file other than look at it as a text file. It was xml but the browsers refused to render it because it was missing the style file (or something like that.)
So skip the download agent and use the manual download.
Download the install zip file first and unzip that file to create a new directory (the normal unarchive step). In that new directory download the common and the maci zip files, and unzip each of them, letting it create the directories for each.
Now fire up the installer, authenticate, go through the install sequence.
If you get a complaint about a .enc file then you probably missed unzipping the common or maci files.