I'm just replying to this post to help people having an empty library when upgrading from FCPX trial to full version because this happened to me and there is no answer to the question. This are the steps to solve the problem.
First you will have two versions of FCPX in your apps folder (FCPX and FCPX Trial) launch the one called FCPX if all you see is a new empty library, right click in the libraries areaand select close library.
Once that library closes a new button that says open library will appear, click that, then click LOCATE, then change the tab on the top of the window which will say DOCUMENTS or FINAL CUT DOCUMENTS and navigate that to your MOVIES folder, select the older library there and voila! your library will move over.
NOTE: this will open one library, if you had more than one library in your trial version select all of them at once while holding the command key to bring all of them at once, otherwise you will have to close one to open the other moving forward.
Once you have all libraries showing, move the trial version app to the trash but don't empty the trash until you're sure everything works by quitting and relaunching the main version.
The two apps are in the applications folder and the libraries are in the movie folder (unless you changed their location) so, by deleting the trial app you're not deleting the libraries.
Hope that helps.
Cheers