Is this D:\ disk a physically separate hard drive in your system? How is your computer currently setup (I'm assuming you are dual-booting and allowed the Ubuntu installer to shift the contents of the Windows C:\ disk in order to make room for the Ubuntu install, let us know if this is correct or not).
Most modern Linux distributions, including Ubuntu, will automatically access other disks that are detected in the system and therefore it should be available by browsing the "Places" area of the window-manager you are using (which version of Ubuntu are you using? They recently switched the default window manager on Ubuntu 11.04 onward and the interface has changed a bit)