The first thing to do is open the project in IJ. You don't need to import it, just go to File->Open and navigate to the top-level directory. IJ will do the rest.
The next thing to do is to build the project on the command line. Just use:
(if you're on Windows, use the "go" batch file instead). There'll be a pause and some bits are grabbed from the Net (about 80MB), and then the build should kick off and run to completion. Subsequent builds won't need to do the download, and should start a lot faster.
If you get stuck, hop onto the #selenium IRC channel or the Slack channel and ask for help. During the week, there's normally someone around who can help.