If your TV's connections are not set up so that one of them is a 'SAT' input then it will not turn on when you press that button on the remote. I think this is all explained in the handbook.
The 'DTV' button does exactly the same as 'SAT' - the remote sends the same message.
I am not sure what output the Nintendo 64 has but I presume you can get a output lead that includes 'component' and will connect to the Avant in the same way as the PS3.
If you do get a STB then it's worth checking if your Avant is fitted with a STB controller (STB-C). If it is then there should be a 3.5mm socket at the back somewhere, to which you connect what some call an 'IR blaster'. Again check the manual for details of how to set up the STB-C, but if it has an up to date chip fitted you should get a STB that is listed so that you can then control this with your Beo4 remote.