For NX Nastran, use the command, Model > Load > Heat Transfer. This will allow you to apply Forced Convection using 1D elements or Flow Tubes to model the fluid flowing across the plate. Please refer to the Femap Commands manual and the NX Nastran Thermal Analysis User Guide.
Note that this is applied to the active Load Set.
Best Regards, Chip Fricke Principal Applications Engineer - Femap Product Development