The distribution file archive (e.g. lev00_2.44.tgz) should be untarred and unzipped, e.g.
lev00 code has to be compiled for every new system/application (contrary to tetr). This is because it requires much more memory (e.g. electron density on a grid) and is written in f773.4.
Before compiling the code for the first time, you should edit the script lev00.comp (supplied in subdirectory /TOOLS/SCRIPTS) to indicate the correct directory for the source code by setting the variable road there, e.g.
set road="/TOOLS"
The compilation of lev00 is performed by calling the script lev00.comp from your working directory (where is your VASP or SIESTA job resides). What is needed is the file param.inc file containing the parameters of your system:
parameter (NGX= 60,NGY= 60,NGZ= 60)
parameter (NSPEC= 2,NIONS= 3)
parameter (NKPTS= 1,NBANDS= 8)
parameter (NPLWV=NGX*NGY*NGZ)
parameter (ispin= 1)
The script copies the file param.inc into the directory where the source code resides (variable $road in the script), compiles the code, and then moves it back to your current (working) directory. After running the script, you should find an executable lev00 there.
In practice, what you should do depends on the code you intend to use, and this will be described in the proper Sections 4, 5 and 6 for the VASP, SIESTA and CASTEP codes, respectively.