I had quite a surprise opening fine the BT5R3-GNOME-32-VM.zip other a 64bit 12.04 Ubuntu and hitting a wall other my 32bit 12.04 Ubuntu laptop.
It displays "End of central directory signature not found" with unzip under linux, that it's only a part of a multipart archive other windows vista and 7 32bit, winrar and windows's extractor.
I checked the archive against it's md5 bca6d3862c661b615a374d7ef61252c5. It was not that.

More surprising when I came back to my 64bit desktop, loading the BTR3 32bit VM I installed file-roller which manages to see the files inside the zip but cannot unzip it o_O

Full error message :
Archive:  BT5R3-GNOME-32-VM.zip
  End-of-central-directory signature not found.  Either this file is not
  a zipfile, or it constitutes one disk of a multi-part archive.  In the
  latter case the central directory and zipfile comment will be found on
  the last disk(s) of this archive.
note:  BT5R3-GNOME-32-VM.zip may be a plain executable, not an archive
unzip:  cannot find zipfile directory in one of BT5R3-GNOME-32-VM.zip or
        BT5R3-GNOME-32-VM.zip.zip, and cannot find BT5R3-GNOME-32-VM.zip.ZIP, period.