I ran one of those machines many years ago... so my memory is a bit rusty.
Which type of ATC does it have? Carousel or Swing-arm?
The one I ran had the carousel type; we would load the tools manually in the spindle using the tool release button on the side of the head (in handle mode IIRC).
Caution: on carousel type there cannot be a tool in the carousel adjacent to the spindle and a tool in the spindle at the same time or it will crash during the automatic tool change. If this is the case, manually remove the tool in the spindle before doing any tool change to be safe. [note: this does not usually apply if it is a swing-arm type as both tools will be swapped at the same time]
1) Call up the tool number you want to load (eg. T1) by typing T1 M6 - ENTER
2) Load the tool 1 manually in the spindle using the tool release button on the side of the head (in handle mode IIRC).
3) Call up the next tool number you want to load (eg. T2) by typing T2 M6 - ENTER
etc, etc,
Note: The tool registry on the carousel type changer should have the pocket # same as tool #... if it is a swing-arm type the registry and tool number may be different even if you reset them, as the tools are changed the pockets and tool numbers will automatically change during the tool change due to the swing-arm swap... be careful!