There have been several releases of the LEGO Universe client. Now, there is only one version (126.96.36.199) that works fully. See the official Downloads Page.
The original client that most users had, which was the version distributed worldwide on CD-ROM discs, will not load past the title screen due to missing files. It was uploaded by Timtech soon after the closure of LEGO Universe. It will not work with server versions 0.3 and above. The original asset files for both Windows and Mac (note that Mac now requires a virtual machine anyway, so just use the Windows client with a VM since the Mac version is missing files) were uploaded by Jon002, and later zipped up by Timtech for quicker download. Timtech also added some audio and .pk files to the client in two more fix releases.
However, after the complete removal of cache.legouniverse.com in mid-2014, the "restored" client would not work on certain worlds (Nimbus Station, Gnarled Forest, and a few others). CDFalcon uploaded a "green light" client in October 2014 to fix these problems. However, it was incomplete, and fixed (by himself) two days later, with a name change as the "Nexus" client.