Hello and welcome ,
First try in easy mode to load DIVX (select expert mode in Divx settings) it wil open in expert mode with fields already filled with divx ac3 2 channel .
Choose 16/9 or 4/3 according to your needs
once in expert mode go to the audio tab and if you want 5.1 just select 6 channels
That should work as audio is already in ac3
Let us know the outcome please ,
Have a nice day
thx for donating