I'm not sure what the problem might be, however I am suspect of the UI.openpanel behaving properly since it uses the Windows '%USERPROFILE%' environment variable in the pathname (not to mention it looks for '\'s and not '/'s). Not sure what Macs do with this. This is the line the error message refers to:
@@imax = (@@index[24,4].unpack('N').first * 2 - 100) / 8
You might try replacing the line that reads:
@@pathname = base.to_s
@@pathname = "/temp/building_0715"
That should at least remove any potential UI.openpanel issues. I really appreciate your trying this out for me.
BTW, if you get that working, Los Angeles has a really nice dataset to play with. The 581MB zipfile can be downloaded from:
Select "HEIGHT" as the appropriate data field.