Noob to Haiku here, though with several years experience of Puppy Linux under my belt. (Used to what the rest of the community tends to think of as ‘quirky’ ways of doing things, so I feel right at home here!) 
I was just about to post on this issue myself, but I’ve been saved the need to.
Followed the instructions; deleted the admin-packages file, re-booted, opened Haiku depot, refreshed the repos, aaaand…stuff installs now.
Thanks, Starcrasher. Clear, straightforward instructions work a treat, every time.
(Been wanting to try Haiku for a long time, but for some reason my old hardware just wasn’t having it. My new HP desktop, however, seems quite happy with it all; so far, I’m liking what I see.)
Keep up the good work, guys.
Mike. 