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First, if your ASCII file is not already named with a .txt extension, rename it that way.

Open Calc. In the pull-down menus, go to File > Open.
Browse to find and select the ASCII file.
Select File type: Text CSV (.csv; .txt). Note: This choice is near the bottom in the spreadsheet file types section of the list.
Now click Open.
In the dialog that appears next, select the Separator options. These are the characters or methods used in the file to separate the fields of data. The same methods must be specified in this box as those used in the file to import the data into a spreadsheet. After selecting the separator type, a preview of the data will be displayed in the Fields section. If the data visually lines up in columns, then the correct separator has been selected. If not, a different separator type may be used in the file. The goal is to match the correct character used as a separator in the file, so that the data will line up nicely in the visible cells. When the data lines up, click OK.

Hints:

The characters used as a separators and delimiters will be visible, if you open the .txt file in Writer and enable the hidden characters.
Click the Help button in the Text Import box accessed above in Step 5 to bring up more information about the parameters there, if further explanation is needed.