Well, in the Add References dialog box you can change "File Type" to look for Access databases (MDB), then browse to the other Access file and choose it. Then you will have access to all it's Public functions.
However, it would probably be a lot simpler to just Copy & Paste the function from the one MDB file to the other. It's more overhead to call the function on another DB, and the reference to the other MDB file will be very easily broken.