Never mind that. At the bottom of your object library, you see the name of your object library.
Mine is named 3DC as you see in the picture.
Mine is stored in the folder 'My documents\3DC 7.1.2.\Media\Objects\'-�, this is the standard place for this file.
Look in this folder for a file with the name you gave your object library.
If it is not there look in 'Program files\3DC 7\Media\Objects'-� or in your main 3DC Folder ('Program files\3DC 7'-�)
If it is not there either, use the windows search function, to find it.
When you found it, also look at the creation date, it should be recent, to be sure you have the right file. Copy it to a save place, and copy it back after the reinstall.