You don't have to donate to download Carbonite.
You also don't have to download to donate. :P
That aside, though, go
HERE and click "Download latest version" on the right hand side. Then you unzip it to where you have your other addons, overwrite any files you're asked for, and you're done!
Do note that the install path is different depending on if your operating system is Vista or newer... or not. For WinXP and older, it's in
Program Files -> World of Warcraft -> Interface -> AddOns
whereas for Vista or Win7 it's
Users -> Public -> Games -> World of Warcraft -> Interface -> AddOns
Hope it helps!