There are two ways to accomplish this that I can think of right now.
1 - Setup a DSN connection and connect to the database as you would any other ODBC data source.
2 - Use OleDB to connect to the Access database. This is the option that I would recommend as it doesn't require the DSN to be configured on the computer where this workflow will run.
To connect to an Access database using OleDB:
1 - Determine if you are using a 32 bit version of Access or a 64bit version. In my case, I'm using a 32 bit version of Access.
2 - In the Output Tool, use configuration drop down to select Other Databases->32-Bit Database Connections->OleDB
3 - Configure the OleDB connection. Select the Microsoft Office 12.0 Access Database Engine OLE DB Provider, input the full path of the Access database in the "Data Source" field, enter the table name you wish to write to.
4 - Once you have configured the connection, set the Output Options to "Append Existing".
See the attached example workflow, database, and screenshots.