1) Make your own forks of l2j_server/l2j_datapack on github (open
https://github.com/L2J/L2J_Server, see the three buttons at the top (Watch/Star/Fork <- click this one)).
*This forking might not be necessary, I'm not 100% sure about it, need someone else's opinion //summon UnAfraid*
2) Open Eclipse, go to Window > Show View > Other..., find SVN > SVN Repositories, select it and click OK.
3) Right-click that panel, choose New... > Repository Location.
4) Go to your L2J_Server fork (
https://github.com/your-username-here/L2J_Server), and take a look at the right side of the page. There are two buttons at the lower right corner (Clone in Desktop, Download ZIP), and above them is your repository's checkout URL.
You should click on the "Subversion" link, copy that URL, paste it in the Eclipse window, and click Finish.
5) Once it is done adding the repository, expand it, right-click "trunk" and choose either Check Out if you want to keep the original project name, or Find/Check Out As... if you want to rename it (for example, if you already have a project named "L2J_Server" in your workspace, you probably don't want to overwrite it).
And after that it should be the usual.