To use this feature you need to do the following.
1. Install Steam. You can download it for free from www.steampowered.com.
2. Add a shortcut to WoW to your steam My Games tab. (Click Games->Add a non-Steam Game to My Games List, then wait for WoW to show up in the Choose Application dialog and add it).
3. Launch WoW through Steam, click the My Games tab and launch WoW. You must launch through here or the steam functionality will not be available in game.
While in game you can now press Shift-Tab to go to the steam menu within WoW, from here click web and it will open a web-browser, just enter the wow-pro url and go to the guide you want. I find if you go to the print friendly version it works much better as you probably wont want to stretch the browser to cover the whole screen.
Voila you can now view a web-browser while playing the game which makes using the guides here incredibly easy. It is also very handy for wowhead, wowwiki pages etc while playing as you can still see your game as you look these up.
The Shift-Tab key combo to bring up the Steam UI causes some issues. Firstly, it is used for a couple of things within WoW, namely switching to previous target and going to previous text edit box (if typing in prices at the AH for example). It also seems to sometimes leave WoW thinking that the shift key is held down after you go back to the game requiring you to tap shift to clear it. Not serious but annoying and would probably drive you nuts if you are tanking. To change the key that is used click File->Settings and change it from the In-Game tab in Steam. I use the Pause/Break key which I never use for anything else and is far away from any keys I do use and it seems to work perfectly).
The browser does seem to freeze every now and then, it seems to work but the view doesn’t update when you scroll up and down. Just close the in-game browser and start it again seems to fix this.