Click on the audio clip that you would like to remove the noise from. Click on the Sound tab on the bottom. Click on the "Noise Reduction" control and select a value up to 100%.
Once you have selected an amount, Mixcraft will attempt to automatically find a good noise sample. Mixcraft shows the noise section by two controls called "Noise Start" and "Noise End".
The above image shows an example of a good noise sample. The noise is determined by the audio section between the "Noise Start" and "Noise End" controls. You can edit the noise sample by clicking and dragging the "Noise Start" or "Noise End" controls. If you want the software to automatically locate the next best noise print, click "Next Noise>>". This instructs Mixcraft to look for the next best noise sample based on what it thinks could be noise. Only you know what noise is, though, so you may need to adjust it manually.