Try to restart Eclipse, then go in File => New => Project… and select a C/C++ => C Project. (http://www.openstm32.org/Creating+a+new+project)
If you cannot find “Ac6 STM32 MCU Project” under the Project type“Executable”, it means the plugins installation has failed.
I just downloaded the lastest version of Eclipse C/C++ then follow installation steps and it works for me. Maybe you should re-install it from scratch.