Jason,
If you pick up an unknown aircraft and it is in the database it will normally populate if you are set up to do this. If you havent set up to autopopulate or the aircraft is not in the database if you open up mylog and press populate it should populate into your local Navdata any unknowns which are now in the server database. If they are not in the databse they won't obviously populate. If you are populating for the last week, you must have mylog open and showing the mylog for the last week, for the last month, the last months mylog etc.
Myself I just prefer to download a new Navdata each day and get the updated database instead of just ones I have picked up. However some users prefer to use populate because of their internet connection being slow and it uses less bandwith than downloading the 68Mb of the whole Navdata.