Starting from an existing eclipse installation, you first have to install Xtext, which is a dependency for Xturtle, from this update site:
After that, install Xturtle from this update site:
During the installation, you will get a warning that the package is unsigned.
You'll know that the editor is working, when you open a
.ttl file and you are asked if you want to add the Xtext-Nature to the project.
Further, on invoking code completion (ctrl-space) in the first line, something should happen...
For more information on features, usage notes and technical background, have a look at the wiki.