It could happen that when completing Visual Studio 2010 Beta 2 setup, the button for launching the off-line documentation installation is not available or that the specific link in the installer start page is disabled.
If you experience this problem, don't worry; it is of course possible installing the off-line documentation later. Select the Manage Help Settings command in the Help menu within Visual Studio:
The Help Library Manager will run; from here you will be able to install the documentation, for example selecting Find Content On Disk:
This last option requires the availability of the installation media for Visual Studio.