How to configure compose key in Ubuntu 14.04

In keyboard settings -> Shortcuts -> Typing -> Compose Key

Click the 'Disabled' across from Compose Key and select the key you want.

You will need to install dconf Editor from the Ubuntu Software Centre so it will appear in your applications.

If you use an applications list it appears under Sundry or if not in your Activities Overview (type dconf in the ~type to search~ box).

There are other ways of achieving your aim but this is a simple gui method not needing terminal skills.

$ dconf write /org/gnome/desktop/input-sources/xkb-options "['compose:{key}']"

where {key} is your chosen key-name (e.g. "['compose:ralt']" would make right alt your compose key.