I thought "PC" was purely audio from the PC
My BEO4 manual (alphabetic appendix) says
"PC Select PC in order to use Beo4 to adjust the sound form a Personal Computer connected to your Bang & Olufsen BeoLink system."
I suspect you need to be using something like V.AUX to pick up an auxillary video source. You can add V.AUX to the LIST function by going therough the normal "add" function.
First B&O (1976) was a Beogram 1500 ... latest (2011) change has been to couple the BL11 with the BL6Ks *sounds superb*