(Sorry, I see Corylea already solved it for you. I'll leave this posted, in the hope that it will help somebody else who isn't familiar with 7-Zip.)
7-Zip is a kind of program called a "power toy". It doesn't register file types or give itself a Start menu entry, so you can't double-click an archive and get 7-Zip to open it, or even launch the program from the Start menu. You launch it from a context (right-click) menu.
You have to right-click the .rar file. This will give you a menu with a number of options. Choose "7-Zip", drag right, and you'll get more options. Choose "Open archive". You'll get a window that looks sort of like a Windows Explorer window.
Drill down until you find a file that looks like "000071 - Order of the Flaming Rose-071.TheWitcherSave" or something like that. It will have the suffix ".TheWitcherSave".
Select that file and click "Copy". You'll get a "Copy To" dialog. Click "Browse" and browse to your Witcher 2 gamesaves folder. It will be something like "C:\Users\Guy\Documents\Witcher 2\gamesaves". Click "OK", and the file will be copied there.
Then launch TW2, and start a New Game. You will get a choice to import a Witcher 1 save.