解决方法：

先手动安装proftpd:

wget http://files.directadmin.com/services/customapache/proftpd-1.3.3f.tar.gz
tar xvzf proftpd-1.3.3f.tar.gz
cd proftpd-1.3.3f
install_user=ftp install_group=ftp ./configure –prefix=/usr –sysconfdir=/etc –localstatedir=/var/run –mandir=/usr/local/man –without-pam –disable-auth-pam –enable-nls –with-modules=mod_ratio:mod_readme:mod_tls
make
make install

也可参见另一片博文：http://phpcn.info/archives/121

然后配置directadmin:

cd /usr/local/directadmin/custombuild
./build update
./build set proftpd yes
./build proftpd

可能最后还需要这么一句命令：

mkdir /var/run/proftpd