Thanks for your questions. The receiver option is only available when the TV detects a compatible device over HDMI. In answer to your other question, the difference between PCM and DTS is a matter of taste: so people prefer one over the other.
To your last question, no, when your TV is in PCM, it will not pass Dolby Digital 5.1 along to any devices. In fact, I believe that any sound source that doesn't originate in the TV is changed into PCM if the audio is output through optical but initially received through HDMI; I believe this is per the HDCP guidelines.
Under Sounds>Speaker Settings>Speaker Select: my options are External Speakers, TV Speakers, Receiver, and SoundShare ...how do I select the receiver option? It is grayed out, I know External Speakers will work but I would like to utilize the Receiver setting
I have another question regarding audio out, is PCM better than DTS 2:5? When in DTS the audio sounds louder, but I'd rather have quality, and does this TV pass through Dolby digital 5.1 when in PCM if I have my Apple TV hooked up to this TV and have audio out optical to my receiver? Please let me know, thank you!