You might have chose wrong option and forwarded for installation. Otherwise there is an option of create custom layout while choosing the type of installation. Just choose create custom layout from type of installation and then you will get the screen to choose on which partition you want to install your OS.
I'm attaching an image of type of installation for your help.
Last edited by Satyaveer Arya; 12-25-2011 at 03:31 PM.