Did you follow the steps in Dual Boot Installation - Windows 8 and Windows 7 or Vista
? If so it should present a Dual Boot menu when it starts to choose which OS to boot.
Did you make sure that you weren't selecting the Windows 7 partition to install Win8 upon?
If so, boot into Win8 to run Repair which should add Windows 7 to the boot menu: System Recovery Options - Boot to in Windows 8
If not, install EasyBCD to Win8 to add Windows 7 to Dual Boot on the Add OS Entry tab. You should be able to figure out at Start>Computer which is the Windows 7 partition so you have its letter if required.