First, locate where the compiler is installed. If it came with C::B, it should be in the C::B's installation folder, if it is not there you can search for gcc.exe in your system disk.
Once you find it, goto "Settings -> Compiler settings", select "GNU GCC Compiler" and the tab labeled "Toolchain executables"; input the path to gcc.exe in the "Compiler's installation directory" control (without the last "\bin" part, for example C:\MinGW) and click Ok.