Download and put the netboot directory content at the root of the tftpd's served /var/lib/tftpboot/ directory:

cd /var/lib/tftpboot/
lftp -c "open http://archive.ubuntu.com/ubuntu/dists/lucid/main/installer-i386/current/images/netboot/; mirror"

# then untar the pxeboot.tar.gz (this step is NOT needed anymore, as of Feisty)
tar xfz pxeboot.tar.gz

Alternatively, if Ubuntu is in your cdrom drive:

mount /media/cdrom
cp -a /media/cdrom/install/netboot/* /var/lib/tftpboot/