To manually mount the Windows partition click on the 'Home' icon on
the Launcher and you'll see the partitions listed using the same names
that you will see in Windows ( Note: not drive letters, only Windows
uses drive letters such as C: D: etc ). Click on the drive to mount it
and then double click the 'User' folder to find the files related to
your Windows login, i.e. Desktop, Documents, Pictures, Music etc...

Once you've mounted the drive then programs in Ubuntu can use 'File /
Open' to locate files on the Windows partition or you can navigate to
the relevant file and double click to open it.

It is possible to auto-mount the Windows partition when you start
Ubuntu if you need but easy enough to click on the drive as and when
you need to once you know how.