because ESET wants the registered username/password before you can download an update. Trial users of NOD32 is the one that can download the defs without username/password and still need internet access.
Their suggestion is connect your PC to another internet connection (sharing internet access) to get the defs.
It's been years since I last have NOD32 so no idea if they have manual updating now. You can try to check in their official forum:
Is there a way for my NOD32 without internet connection to be updated? Thanks! =)