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Choose Track > Show Master Track and the Master track will appear at the bottom of the track list in the main area.
You can automate it's volume level... but be careful that the level meter (in the bottom tool bar) doesn't ever go into the red. This will result in clipping and distortion of the audio signal.
You can also try adding an effect on your Master track. Try using the included AUPeak Limiter to boost the levels. Again, listen carefully to the changes you make so that you don't end up boosting the volume too much and therefore creating a distorted or clipped audio signal.
Hope this helps