As MarkE19 says,
Make sure you have installed flac, you can get it from
here After you have installed flac start EAC and press F11, this will open the compression options window,
select the external compression tab,
check the box marked "use external program for compression"
select User Defined Encoder for the parameter passing scheme,
browse to the path where you installed flac and select the flac.exe file
I also check the box marked "delete WAV after compression" just to keep things tidy
click ok and you should be good to go
Bryn.