To get started click the Multiview link in the sub navigation menu.
Next click the 'Add more forums now' link.
This will take you into your account settings page and the multiview option, you can also get into you account settings by clicking on your name in the navigation menu.
All you have to do the is type in the forums you wish to add into your multiview.
Once you have selected the forums you wish to include all you need to do is click the multiview link to have all posts in date order of the forums you specify. To enable the quick creation from your multiview page there is a 'Create Thread' link.
This will open an overlay in order for you to choose which forum you would like to create a new thread in.