There are 3 common things that can cause the product pick box to be grayed out at the product install screen.
1) Internet Explorer 7 or higher is not installed
Fix: Install IE 7 or higher.
2) DWF Viewer is installed.
Fix: Uninstall DWF Viewer and try reinstalling Revit.
3) The product is already installed, or was uninstalled incorrectly.
Fix: Verify the product you want to install is not already installed. If it was and had been uninstalled, make sure you went though the Clean Uninstall process (TS14633513).
In addition, if you have Windows XP 32 bit, please also verify SP3 is installed.