If you want to import several data files, the first parameter Import each file as must be set to "New Table".
Concerning the separator which is used between the columns, you must bear in mind that there is no grouping of separators, so if you use "SPACE", you must use only one separator between each column or check the Simplify white spaces option.
If your data files have a particular structure, for example if they present several lines containing the description of your experiment, before the actual data start, you have the possibility to skip the n first lines of the file. You can also skip all lines starting with a comment character/string. The comment string can be customized via the text edit line Ignore lines starting with.
If you choose to use the first line as column names and/or the second line as comments to be displayed in the table header, you must use the same separator between the column names and between the data columns.
Finally, assuming that the data files you want to import were created using different decimal separator conventions than the ones you're using, like for example a comma character for the decimal point instead of a dot character, you have the possibility to convert the data to your current settings, by specifying the Decimal Separators convention used in the ASCII files in conjunction with the Omit thousands separator option.