Okay so you have an Attachment button on a Form within your Action Bar but you do not know how to add the pop up window to add the file to the Record associated with that Form.
Is that correct?
If that is correct, before you go coding a whole bunch of things. I am quite certain that someone created (and/or was included as part of VB at some point in time and I'd surprised if it is not in the current version of VBA) a "File Lookup" Form/Module. I do not have time to look it up right now but I believe that is what you are looking to implement.
If the predefined Form/Module cannot be found then you will have to create it from scratch.
Basically the button is going to call a modal pop-up window to allow the user to select the file. This will then return or set the full filepath and filename for the file to be attached. Now personally I would not "bind" that field to the database at all as adding files to a database is a bit touchy even on SQL Server so I can only imagine that it will have similar issues on Access. Also this file should be contained within a File table that only contains a uniqueidentifier and the file. Then referenced in a table that contains the filename associated with the file along with whatever else you want in that table. This is due to the fact that binary files tend to cause database table defragmentation and, as it sounds, is not a good thing to have happen to your tables.
If you do not find this on your own let me know and I will see if I cannot dig it up some where. Or if no one can find it and you are not sure you can create it then let me know and I will see if I cannot help walk you through it. Hopefully the former will be viable as that will save a lot of coding.