Select the "Enter check distances manually later" to prepare the Imports and Coordinate Systems Pane:
You will later define one or more known distances between two points (after you click OK on this dialog). See also Add new empty object.
Select "From a file..." to browse to a file containing a set of distances:
The distance data will be extracted from the file and the Imports and Coordinate Systems Pane will be set up to assign distances to point pairs (or lines) in your model. You must also set the units of the data in the file. See Import File Types for a list of supported file formats.
See Check distances.