As I can enter a Alternate Airport into the FMC

No thats only possible in Airbus Style FMGC's...On Boeing's - when you divert - you have to enter your alternate on RTE Page LSK 1R and then insert the new Routing on LEGS Page.Hope this helps,Matthias

In the 747/767 you can program a RTE2 with the alternate.

Hi,In the real aeroplane you can set up Alternates via a Missed App or Direct from present position and you will get fuel, recommended CRZ alt etc but to actually fly an Approach at an Alternate you must, as rightly stated above, select that airport as the DEST on RTE page 1.I don't think this facility is implemented in the PMDG however.ATB,