MecaLug follows the design methodology provided in ASME BTH-1, ' Below the Hook Lifting Devices'. You enter the actual weights that you expect to be lifted, without applying any impact factors. Then you select a Design Category from the ASME BTH-1 criteria that corresponds to your lifting situation. An excerpt from ASME BTH-1 is provided here:
Based upon the category specified, then the safety factor (Nominal Design Factors) are applied according to section 3.1-3.1 (Shown above). For example, if Category B is selected the the safety factor of 3 is applied in the design. Since the analysis is based upon Allowable Stress Design (ASD), this safety factor of 3 (Nd) would be above and beyond any other safety factors that are inherently present in the ASD design.
You do not need to include the traditional "Impact Factors" to your loads entered into MecaLug, unless you want to be ultra-conservative. The Nd parameters already take into account the normal safety factors for the category of lift being performed, and applying an impact factor in addition to these safety factors will only add that much more safety to the analysis.