You have 2 options (at least):
(1) In the database windows,click NEW and create a Linked table which you point at the text file.
The text file will then appear to act like a native Access table
(2) Import the data into an existing table. Goto File-Get External Data-Import
Option 1 is great if you have a well behaved and formatted file. Also has the benefit of allowing you to substitute a new text file and get a new table
Option 2 is best for retaining the data. HOwever, you will need to ensure uniqueness, so you are probably better off using an intermediate query to load you main table