Open a ticket and explain your situation. It helps if you can discuss taking it over with the previous author through messages on this website, then you can add one of the admins to your conversation and they can read your discussion with the previous author and easily verify permission. Be sure to explain to the previous author you plan to do so, etc. I did this when I took over maintaining BaudMark addon for WoW.
If you can't get a hold of the previous author then ask a mod what your options are.