Remote application start
The screen sharing button you mentioned is located in Finder. If you turned on Back to My Mac on your iMac the iMac should show up in the Finder's sidebar. Click on the iMac's icon and you get the screen sharing button.

Alternatively if you know the IP address or URL: In Finder press commandshiftg and type in /System/Library/CoreServices/ to navigate to the place where the screen sharing application is located. Open screen sharing and type in the address of your iMac and connect.