Thank you for your post.
When you click "Add" in the custom ranges box, another box appears. In the large scaling box you enter the values for scaling so that the original value appears on the left (in mV) and the scaled value on the right.
Therefore, if you want 0V to be 0A and 5V to be 10A then you enter:
The software then uses these values to work out the points in between. You can add multiple entries. You put the units for the scaled value (in this case A) in the box at the bottom.
Hope this helps