Let me make sure I understand what you're asking: You have some data that comes to you as straight text, you import it into Access, and then you need to parse out the day of the month?
As long as they always come to you in a format that matches your computer's settings (i.e. MM/DD/YY or DD/MM/YY, not both interspersed), this is fairly simple.
When you go to Import your data from the text file, click on Advanced and create an Import Spec. You will only have to do this once and then you can use that Spec on future files. Format the date fields as Date/Time and then use the Day() function to extract the day of the month for your purposes.
If for some reason you can't do this, try using Day(CDate([TextDateField])) but that is a kludge and I wouldn't recommend it if you can get around it.
HTH,
David R