To start CLUSTER (clustern in linux, clustern.exe in windows) unpack the files and place them in a selected catalogue with path to this catalogue replacing the first line of the current.cfg file.
Most of the program options is intuitive or explained in the bottom hint line after placing cursor on the option name.
The program is a compilation of publicly available procedures (Fletcher-Reeves minimization, leap-frog molecular dynamics in microcanonical ensamble, Monte-Carlo Metropolis) and authors algorythms (e.g. related to diffraction, PDF etc.).
The compiled program after choosing option Builder--> New cubooctahedronn and setting e.g. 55 atom Au cubooctahedron should show the structure (yellow spots on black background) that can be rotated with the mouse wheel in direction of the cursor. To deepen the space-feeling one can fill field (edit line) 'Treshold energy Ev' e.g,. with value 2.7 (eV) and click the button 'Show bonds' (for the display panel refresh click onto black window field). The interatomic bonds are then displayed with the nearest bonds yellow and the farest light blue. Structure can be minimized energetically (relaxed) by clicking  Minimize --> Relax using  'Conjugated gradients' option and 'Minimize' button.
Molecular Dynamics at temp. 300K usually quickly transforms this structure into icosahedron, that , for such small clusters have lower energy. One can stop (MD, STOP) this procedure after transformation and minimize the cluster (Minimize --> Relax) to obtain symmetric cluster with 12 5-fold symmetry axes.
The potential scheme of the program can use several interaction potential with default - Sutton-Chen N-body potentials described in Phil.Mag.Lett. (1990) 61(3), 139, that realistically describes cohesive properties of metals.
For each structure one can calculate powder diffraction patern of randomly oriented set of such separated clusters (Analize --> Powder Diffraction Pattern) or diffraction pattern of this cluster oriented as on the program display (Analize -->Diffraction with SV in z) (SV stands for scattering vector). Similarly the Pair Distribution Function (PDF) can be displayed with (Analize-->Radial Distribution Function). Most of other options is intuitive and worth trying. Some Builder options use parameters defined in (Analize --> RDF and DFR setup).
This short guide serves as a minimum intro to further explore the program. More exhausting description is in preparation.