I think you may need to find the IP of the server. A popular way to do this with other people is to download Log Me In Hamachi and the Haguichi GUI (atm Hamachi is commandline only for Linux)
Go to https: // secure . logmein.com / US / labs/, go down to Hamachi for Linux, download the .deb, install through Software Centre as normal
Download haguichi by doing this in the Terminal
sudo add-apt-repository ppa:webupd8team/haguichi
then
sudo apt-get update
then
sudo apt-get install haguichi
then click the ubuntu icon at the top of the screen and search for haguichi and launch it.
After you have done this
Click Connect
press f2 and note the ip address.
Launch the MC Server .jar
Type the ip address you wrote down into the Multiplayer tab of Minecraft.
Hope I could help,