Memento Composite imports .csv database files exclusively. CSV (comma separated values) files are best able to support a variety of character types (accents, apostrophes, other non-alphanumerics) than other files types.
If you have been given a .txt or other type of file for your database, open it in Excel or another spreadsheet-type software.
Your data will be displayed in a grid.
Follow the formatting shown in the supported file types and formatting page to correct your index file, if required.
When the work is completed, choose File > Save as... in your spreadsheet software then resave the file as a .csv as your chosen file type.
Save it next to your original database file under a slightly different name (eg. index2.csv). This will make it easier to spot when you are importing it into Memento Composite.