how to set parameters in ndt_mapping while my velodyne is slant to create 3D Map

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I am assuming you mean the transfrom between the velodyne and the base_link (vehicle coordinates). You can use the GUI (roslaunch runtime_manager runtime_manager.launch) to set the ros parameters that NDT mappings uses for the transform between the sensor and base_link..

or you can just set the rosparams in a terminal using "rosparam set param_name value". The parameter names are: tf_x ,tf_y, tf_z, tf_yaw, tf_pitch, tf_roll. These could be set in a launch file also.