I've installed windows 10 and Ubuntu 15 on my laptop, but I don't get the dual boot menu. Ubuntu installation appeared successful, but when I turn my laptop on, it goes straight to Windows without giving me the option to choose ubuntu. It's a brand new laptop, so still don't have any data I'm worried about losing.
Go to your BIOS settings (press Esc, F2, etc. it depends on your manufacturer) and check the boot order. 'ubuntu' should be one of the options. Set it as the top boot priority. If it's not there, grub may not have installed correctly.