GeoRaptor doesn't yet work with SQLDeveloper 4.* but it does work fine with 3.2.*. I don't have version 3.1.07 to test with - I am using it with 3.2.20.09 which is the last SQLDeveloper 3.* release. If you're having difficulty installing it on 3.1.07 then I'd recommend updating to the SQLDeveloper 3.2.20.09 release.
Ensure you've got the latest GeoRaptor jar file (release 3.2.3.0307) from: http://sourceforge.net/projects/georaptor/
In SQLDeveloper, go to Help->Check for Updates...
Select "Install from Local File" and browse to the GeoRaptor jar file. You'll need to restart SQLDeveloper, but that should be it. GeoRaptor should now appear at the bottom of the SQLDeveloper "View" menu.
Let us know how you get on,