On a base install of Ubuntu 14.04.2 LTS, simply drop the message you want in:
/etc/motd
By default, the MOTD will appear at the end of the other dynamic MOTD content. For example, here's the output from a Vagrant ubuntu/trusty64 (v20150506.0.0) box with a /etc/motd file added:
$ vagrant ssh
Welcome to Ubuntu 14.04.2 LTS (GNU/Linux 3.13.0-52-generic x86_64)
* Documentation: https://help.ubuntu.com/
System information as of Thu May 14 20:06:56 UTC 2015
System load: 0.39 Processes: 78
Usage of /: 2.8% of 39.34GB Users logged in: 0
Memory usage: 24% IP address for eth0: 10.0.2.15
Swap usage: 0%
Graph this data and manage this system at:
https://landscape.canonical.com/
Get cloud support with Ubuntu Advantage Cloud Guest:
http://www.ubuntu.com/business/services/cloud
0 packages can be updated.
0 updates are security updates.
#####################################
Howdy! This message is from /etc/motd
#####################################
Last login: Thu May 14 20:06:56 2015 from 10.0.2.2
vagrant@vagrant-ubuntu-trusty-64:~$
That's all there is to it if you're working on a system with the default settings.
Some other troubleshooting notes:
Using "/etc/motd" is a change from earlier versions. Recommendations for 12.02 were to use "/etc/motd.tail". That doesn't work in 14.04.2.
Other recommendations I've seen include changing some values in "/etc/ssh/sshd_config". In case those have been altered, here's the defaults that work:
UsePAM yes
PrintMotd no
Likewise, here are the defaults for the /etc/pam.d/login I've seen referenced:
session optional pam_motd.so motd=/run/motd.dynamic noupdate
session optional pam_motd.so
That's everything I've seen referenced for troubleshooting.