Zagreb City - Tourist Office:
http://www.zagreb-touristinfo.hr/?l=e (English pages)
How to Get There
By Air
Zagreb Airport is located 17 km from the centre of the city, or 20-25 minutes by bus.
http://www.zagreb-airport.hr/en/
The Zagreb Airport bus terminal (bus stop) is at the Central Bus Station on Marin Drzic Avenue. Goes every 30mins.
By Train
The Main Railway Station is located in the centre of the City (at Kralj Tomislav Square 12, a ten minutes walk from the central city square).
International schedule:
http://www.bahn.de/international/view/en/index.shtml (Note: the schedule changes at December 15, so it's not the March schedule).
There are daily trains from Ljubljana (2h), Vienna (4h), Budapest (5h), Belgrade, Venezia (night train, 8h)...
By Road
Main international roads are: Trieste-Ljubljana-Zagreb; Graz-Maribor-Zagreb; Klagenfurt-Ljubljana-Zagreb; Budapest-Varazdin-Zagreb
The Central Bus Station is located on Marin Drzic Avenue, a few minutes by tram (line number 6) from the central city square.
http://akz.hr/En.htm