Are you using the wizard? If not I recomand it to you as this will get you through all the steps needed to secure your database.
This is a list of these steps. If you have a problem somewhere, specify which step you have problem with and what you have done so far.
-Create the groups.
-Create the users.
-Assign groups to each user.
-Complete the .mdw file.
-Create a link file that would open Access, then the database, then the .mdw file.
I hope that this could lead you into the right path.