From Hanover Airport to Hanover Main Station:

Take suburban train S5 ( Hameln Bahnhof /  Paderborn Hbf) from Langenhagen Airport to Hanover Main Station.

 

From Hanover Main Station to Mercure Hote Hannover: 

Take the underground 1 (Laatzen), 2 (Rethen) or 8 (Messe/ Nord) from Hanover Main Station to the station Aegidientorplatz. Alternatively use the tram 10 or 17 (Aegidientorplatz) in front of the Main Station.

The hotel is within walking distance from there. 

Go along Friedrichswall, then turn to the left side to Willy - Brand - Allee. The mercure hotel is on the left hand side.