Hi Timmy, welcome to EuroHarmony

First thing's first - you will have to go to the ProPilot page and log in. From the main page, go to Divisions -> ProPilot and sign on with your four-number pilot number and your password (keep in mind the ProPilot page is only available for Internet Explorer at the moment).
Then, at the top, click on Lock Flight and put in some details about the aircraft you're looking for. When you've found a plane you want to fly, click on its registration number. In another window, this will open all the flights available to the plane. Click the one you want to fly and then click Lock Flight.
After you've done that, you've locked a flight! All you need to do is go back to Flight Logger, put in the details about the flight you've chosen and click the Lock Flight button.
However, you only need to lock flights if you intend to fly on our ProPilot system! For a 'regular' offline flight, just select the aircraft and the flight you want to use in the Flight Logger and start up FS.
Hope that helps!