Assuming you've got an audio interface that allows you to record more than two tracks simultaneously, first make sure you've got the latest driver installed from the manufacturer's download page.
Once that's installed, go into Mixcraft's Sound Device preferences and select ASIO mode.
Click on the small arrow next to the Arm button on an audio track:
Arming option 1 on the left gives you input 1.
Arming option 1 on the right gives you input 2.
Arming option 2 on the left is input 3.
Arming option 2 on the right is input 4.
And so on...