I don't have Samplitude Pro X, only Samplitude Music Studio (SMS), but I believe that the process is similar.
In SMS, you can bring up the mixer, set up everything, and then do the mastering on the right hand side. See the first image below. You can open the Mastering Suite for fine-tuning. See the second image. Then if that is all that you want, you can go to File, Export, mp3. All of the Mastering effects will be in the mp3, but you are not mastering for a CD with this method.
Note in the mixer the button at the bottom of the Mixer to mix the song into a wav file. This would then be used for final mastering/CD mastering.
For more information about Samplitude Pro X, I suggest that you post your question on the IPS Community board where you have all of the experts. Also, you can look at the video tutorials by Kraznet. is one about CD mastering.