there are a few ways to ADD an Audio track to your arrange page...

Option 1:

Launch Logic > in the Menu inside the Arrange window you have an option of FUNCTIONS
which brings you a drop box, where you will find the option of TRACK -
this will bring you a side menu where you have an option of CREATE or CREATE with NEXT Instrument...
this is where you add your track -
If you want a track identical to the one you have highlighted then just hit CREATE
If you want one that is adding to the current.. ie...if you have highlighted TRACK 8 and you want to add TRAC K 9 then hit CREATE with Next Instrument

Option 2:

highlight the last Audio track (track 8 in the default song)
and hit CONTROL - SHIFT - RETURN