Adding waypoints to airport

I am getting very confused. I have the PMDG 737-800 with the FMC and I was surprised to find that it has no departure waypoints for KPHL. I downloaded some waypoints which included some of the waypoints for KPHL taking off from runway 08. I looked in the PMDG folder and found a folder called SIDSTARS which had KPHL listed but did not have any departure waypoints listed. I type in one of the waypoints called MIFLN including the lat/lon information exactly as the other waypoints were listed. I then went to the PMDG 737, opened up the FMC and when I entered MIFLN it says not in the database. I know I am missing something but I don't know what. I am really surprised that the departure waypoints for KPHL are missing. It seems to be a simple matter of entering the waypoint and typing the lat/lon and away you go but this is not the case. Perhaps someone out there can stear me straight as to what the FMC is looking for and where it is.

Many thanks