"Add" Network Neighborhood? That sounds like a RELATIVE impossibility. It should be there by default.
When you click on the Open button, I assume you get the standard dialog box.
This dialog box should show the files and directories in the current/most recently used path that Access knows about. When you click on the pick list for "Look in:", doesn't Network Neighborhood show up (near the bottom) as the list scrolls?
You might also try the Up One Level button (yellow folder with the upward pointing arrow). Click that several times and it should eventually get you to the My Computer/Network Neighborhood level.
If your open dialog box always defaults to the same directory, you might try creating a shortcut on your desktop to Network Neighborhood and then moving that shorcut to the directory that Access is pointing toward.