Dubrovnik airport is not well served except for the local bus into Dubrovnik. I have heard of people waiting out on the road to stop the Montenegro buses coming through from DBV but can be risky.
I have taken a Globtour bus from Montenegro to Dubrovnik in 2010. It leaves daily at 1500 from Kotor. The driver allowed us to get off at the airport (however, we later learnt that someone getting off at the airport took the wrong bag, so they may not be willing to do that again).  I can't locate the route on Globtour's website but by following a link from Globtour, I could book it at adriatouring.com/BookingStep1.aspx
You might also like to look at libertasdubrovnik.com/ and follow the link to the right 'Sorted by company'  (libertasdubrovnik.com/voznired2.pdf) and you will see bus services between Dubrovnik and a number of points in Montenegro. Herceg Novi is the nearest point to the border crossing and from there you can get buses to various points in Montenegro. You can follow up some of these companies on the web, e.g. Blue Line (blueline-mne.com/internationaltransportation.html)
If there are a few of you, taking a taxi is probably the easiest.
Enjoy yourself. The hassle will be well worth it!