To use the mdata file output by the mesh card in MCNP software, first convert the file using gridconv.exe, which formats the data for MCNP. The converted data includes x, y, and z coordinates in the first three columns, followed by corresponding values for each mesh grid point. To create a section graph for the xyz axis, plot the data using graphing software, connecting data points for visualization. If a more detailed graph is needed, adjust the mesh grid in the input file and repeat the conversion process. For further assistance, consult the MCNP user manual or contact software support.