Click on the midi track number, As shown below.
It it will turn yellow
To assign the ‘Vst instrument’
Left Click the box the arrow is pointing at.
Choose Vst instrument.
For Vst Instruments that have several instruments assigned to different channels.
If you have setup your VST to have multiple instruments on channels 1-16 then you can assign a channel to this midi track.
Left Click the box the arrow is pointing at.
Now choose the relevant channel to the instrument you wish to assign.
Remember Channel 10 is for the Drum Track.
To set up Synth please read ‘Synth Setup’ – in Vst Instruments section.