Get the latest version of Skype from www.skype.com.

In Skype, open "Tools" > "Do More" > "Get Extras".

Find WhiteBoardMeeting extra in Extras Manager. Click open button to start downloading WhiteBoardMeeting installer.

Wait until Skype downloads installer.

Follow instructions on screen. Make sure you read the EULA.

If you are prompted, install latest .NET framework available for your operating system. If you already have .NET framework 2.0 or later, go to next step.

Installer installs WhiteBoardMeeting and associates .wb file extension to it.

Select installation directory.

Wait until WhiteBoardMeeting installation finishes.

Click Close to exit installer

You can now start WhiteBoardMeeting from Skype's Do More... menu or from Start menu.
NB! If you do not allow WhiteBoardMeeting to access Skype in a Skype popup that appears on first startup, you will not see any participants nor be able to collaborate. To fix this, open up skype main window, go to Tools -> Options -> Advaned. On the bottom should be a link to manage programs' access options. Click on it, search for WhiteBoardMeeting and make sure it says "Allowed" underneath it instead of "Denied"