That’s because Microsoft Access isn’t a client server database.
You’ve got a couple of options.
1. Setup a second access database which has linked tables pointing to the current access database. The user then downloads the second database and when they make changes those changes would then be uploaded to the main database in real time. I’m not sure how well this would work however.
2. The better option would be to transfer the database from Access to the SQL Server which comes as a part of SBS, then have the users connect to the SQL Server database (this could be done via Access and linked tables) or create a new web based front end to allow the user to do the data entry via a web page.
As I said above, #2 is the better option as SQL Server is a client server database, and it’s designed for many people to be using it at once to update data.