You will be presented with the dialog shown below after selecting the Import from File import method:
Importing adds all computers specified in the import file to the current list of computers, the file should list one computer name per line. Existing computers are not removed from the list.
Note: If the import file does not contain group specifiers (see File Format in "Exporting") then all computers of the file are added to the currently selected group. If the file does contain group specifiers then only computers from matching groups will be imported.
After you have selected the file to import from click Next to see the summary screen: