hi,
 i was flying on Vatsim yesturday and i was using the FMC so i had the LNAV and VNAV buttons on but when the controller told me to turn left heading 190 i couldnt cause fmc was just fallowing the path. so i tried ingaging the hdg button but i couldnt figure out how, in the PMDG747 you just click the button and it will turn, in  a default fsx 737 all you have to do is click the HDG HOLD button. i tried both with no luck, so how can i engage the HDG button?

-thank you