This is an error you make see when you're recompiling the kernel, particularly for the first time on a new machine. FreeBSD expects folks to have installed the source as part of the installation and to be compiling as root. If you've done that, the directory should exist and you should have permission. Of course, we don't always compile as root (it's a bad idea!) and we do compile on machines we didn't explicitly install the source on, accessing it through NFS instead.
The solution is to:
% su % mkdir /usr/obj/usr % chgrp src /usr/obj/usr % chmod 775 /usr/obj/usr
This should allow you to proceed.