Looks the the dmd.2.031.zip has the osx directory as a single file.

When you try to compile the samples/d/listener.d, you get this:
$ dmd listener.d 
/Users/mikekinney/dmd2/osx/bin/../../src/phobos/std/socket.d(79): Error: module socket cannot read file 'std/c/osx/socket.d'

Checked the zip file and it does not show dmd2/src/phobos/std/c/osx/socket.d :
$ unzip -l dmd.2.031.zip | grep socket.d
    15145  05-03-09 15:07   dmd2/src/druntime/import/core/sys/posix/sys/socket.d
     2694  07-05-09 01:19   dmd2/src/phobos/std/c/linux/socket.d
    38820  07-05-09 01:19   dmd2/src/phobos/std/socket.d

Because osx is a file not a directory:
$ ls -al dmd2/src/phobos/std/c | grep osx
-rw-r--r--   1 mikekinney  staff  2633 Jul  5 01:19 osx
$ head dmd2/src/phobos/std/c/osx
/*
    Written by Christopher E. Miller
    Placed into public domain.
*/


module std.c.osx.socket;

private import core.stdc.stdint;
public import core.sys.posix.arpa.inet;