When I am installing Ubuntu 16.04 as a dual boot alongside Windows 7 I cannot add partitions. Maybe there are too many disks.
Have you tried shrinking your C or D drives? To do this:
- Right-click the drive you want to shrink
- Choose Shrink
- Set the amount to how big you want your new partition to be
Once you've shrunk it, right-click on the black "Unallocated" area and press Create simple volume.
Alternatively the Ubuntu installer (if booted from CD or USB) should offer you the option to install Ubuntu alongside Windows and you can set how much space you'd like to allocate to each from there.