If you want to do the 'proper' thing you should automate the faders on the mixer to fad the audio in. This may take a bit of practice at first, but basically you will set the fader all the way down at the beginning of the song, enter into record automation mode, and raise the level as you wish.

You can also toy with the velocities will to simulate a fade in, as kings said, and this may be a bit simpler if you aren't familiar with automation, but I wouldn't recommend bouncing your instrument to an audio file to fade it in as it will reduce the quality of the final render.

Send me an email at davidks107@msn.com if you need more detailed instructions.