Michael,
You need to have a ssh public key (As explained by Endre above)
If you already have a ssh keypair then :
run " cat ~/.ssh/id_rsa.pub" on unix terminal
You will see output as explained by Endre above.
Copy the entire output (unless off course there is an error) into clipboard and paste in the Edit Box titled "Public Key" (Part of Import Keypair" pop-up dialog on Openstack Dashboard) Give some name to this keypair, and click submit
If you do not have a ssh keypair then :
On linux machine you can run "ssh-keygen" command. If you are unfamiliar with this command then you can "man ssh-keygen" (Or look at http://www.manpagez.com/man/1/ssh-keygen/ (http://www.manpagez.com/man/1/ssh-key...) )
By default you can just execute "ssh-keygen" command and go with all defaults. (It is interactive command) At the end, two files id_rsa and id_rsa.pub will be created under ".ssh" folder in your home directory (Remember dot in the beginning)