Select the "From scale file..." to set up the Imports and Coordinate Systems Pane for multiple scales (after you click OK on this dialog) defined in the imported file:
Click the "..." button to browse to the file and select the units of the data in the file.
A scale file must contain at least two columns: the id column and the scale distance column. See Import File Types for a description of required data and a list of supported file formats, and Setting Scale for information on assigning the scale to points in your model.
If using offset target pairs, multiple scales can be set automatically - Define Scale/Checks Between Offset Pairs.