It should be simple.
1 - press Y ( for system audio properties)
Make sure you have the correct driver. Generally you should use ASIO if possible for best performance.
In the menu choice Audio Devices, be sure to check all relevant input channels.
2 - in the menu, Track / Insert new tracks / Add one track
do this as many times as you like to in order to have enough channels.
you migth need to scroll and/or decrease height of channels
3 - for each channel, select input source and if mono or stereo
I preferr the track options menus, try Alt-I (but there are are other ways, ie right-clicking the little loudspeaker symbol in each track).
The little >> button goes to next track.
Note that all sound cards handles stereo tracks as odd/even, you cannot do a stereo pair from even/odd.
Be warned, there is a bit of learning curve as Samplitude is one of the more competent partners you can find in this business.
// Gunnar