Mixcraft doesn't include dithering because the creators of Mixcraft feel it isn't necessary to obtain the highest quality audio. Dithering works by introducing random noise to the audio, which may or may not cover up any rendering artifacts. There may not be any rendering artifacts in the first place and you'd just be adding random noise for nothing.
If you're hearing obvious artifacts in your mixed down audio, the cause is likely something else and we can help you figure out what that might be.
That said, you can always try third-party solutions to see if you like the results.