This is the original implementation of lDDT, from 2013.
You should instead be using version 2.0, incorporated into the Structure Assessment tool!
Download the latest lDDT version
lDDT is developed within the OpenStructure framework. You can get and compile the latest version of OpenStructure/lDDT by following the instructions at this link. This includes recipes to build images for Docker and Singularity to simplify the installation.An online version of the lDDT executable documentation can be found here.
Since version 1.8, OpenStructure also includes a command line tool to compare structures which goes beyond the capabilities of the lDDT executable.
If you use local distance difference test (lDDT), please cite the following reference:
Mariani V, Biasini M, Barbato A, Schwede T
lDDT: a local superposition-free score for comparing protein structures and models using distance difference tests.
Bioinformatics 29, 2722-2728. (2013) 
23986568
10.1093/bioinformatics/btt473
2398656810.1093/bioinformatics/btt473