I havent installed Hamachi myself but from what I read I'll try and answer as best as I can.
1.If you're not home and need to access your home PC you'll need to install Hamachi on the source PC.So for eg. If you have a laptop and are in China and want to access your PC in the US you'll have to install Hamachi on your laptop. While this might sound strange its the way Hamachi works.
PC A ---> PC B I believe is actually split into:
PC A --> Hamachi Server
PC B is permanently on in the US
The Hamachi server negotiates the connection so that you now take a path:
PC A --> Hamachi Server --> PC B instead of a direct PC A --> PC B
Not really sure about how exactly Ssh or VNC would work though with Hamachi.
You might want to ask around here:
Finally you might also want to read these posts a bit carefully. There are people here who believe its not totally secure.