Problems installing Gallery Extension

I’m trying to use the Gallery Extension:

but I can’t seem to get it up and running.

I have the files unpacked in my vendor/extensions directory, but when
I try to initialize it like this:

rake radiant:extensions:gallery:install

I get this message:

Extensions cannot be used until Radiant migrations are up to date.
rake aborted!
uninitialized constant GalleryExtension

I’m completely new to Rails, so I don’t know what might be tripping
this up. The “Galleries” tab does show up in my Radiant admin area,
but clicking it generates a 500 error.

I found a similar thread here, and tried the suggestions
(reinstalling Radiant, reinstalling the Extension), but can’t seem to
get it working.

Any help appreciated!