Even though I am German I traveled many years by train and its really easy to do. Even if you come early like Monday morning its really not an issue. Just use the normal train from Hamburg central station to Itzehoe.
What I can recommend is to wait at Itzehoe train station. Instead of using a bus to Wacken you can ask another group that goes by Cab/Taxi to include you and split the bill. Thats about as cheap as the bus but more comfortable. Theres usually bigger cab cars waiting there taking groups even as early as monday.
If you stay in hamburg go to Nightlight Hamburg bar you can't really miss it as its in main "Reeperbahn" party area
http://www.nightlight-hamburg.de/anfahrt/. Theres a big horde of Wacken fans partying there any evening before Wacken and you can probably find a group you can follow to the campsite the next day and if you have questions regarding Wacken you get all the answers and help there too. But from Hamburg train station its no issue either just follow the black shirts.