Wolfenbüttel is served by two international airports: Hannover and Leipzig. It is also close to Braunschweig Aerodrome. Hannover is the closest and there are flights from most parts of Europe and the US.
To Leipzig there is less choice but cheaper flights!
Whilst Deutschebahn is fabulous it is probably much more practical to hire a car from an airport and drive. If you have a Sat Nav then you will be set but here are some map routes for your consideration.
Hannover to Wolfenbüttel
Leipzig to Wolfenbüttel
note: we would always recommend taking the ‘via Magdeburg route’ since this is much quicker and there have been lots of roadworks/baustelle on the A395 in recent times…