By it's very design, Hostblock is independant from the particular browser. You don't need a browser at all to use it, but you can also integrate it into any browser you want.
First, you have to get Hostblock. The installer places it in %programfiles%\Internet Explorer\Plugins\Hostblock\Hostblock.exe. You can get Hostblock portable aswell and place it just wheretheheckever you want.
Second, you have to do the really difficult step and integrate it into the browser. It's good to know what to do with Hostblock.exe to make it block something. Well, it couldn't be easier than that, just call Hostblock.exe and attach the URL of whatever
you want to block as the first argument. You don't have to parse the URL by yourself, you can feed Hostblock with the full URL of let's say an ad image or a Flash control, it will extract the host address by itself.
I recommend adding a context menu item for the site itself, for images and for links. But it's really up to you.
I can't provide support for the most difficult part, namely how to create the context menu entries and how to make them call Hostblock. Refer to your browser vendor for further details.
As for Opera, you might want to click
Hostblock in Opera