ATmega2560 custom board problem

bigal83:
If I gave it the path to the one in arduino (I only have AVRdude through Arduino) it stood for a few second and than gave me the error "stk500_getsync(): not in sync: resp=0x00". What am I doing wrong here?

Looks like you may have specified the wrong COM# port. It should be whatever your Uno show up under in the IDE Tools->Serial Port menu.