Use the "import" option from the "simulation" menu.

When you open G3 and are looking at the runway with your airplane ready for takeoff, there will be a menu bar across the top of the screen. The first button up there is "simulation". Click on it and look at the dropdown menu for "import".

When you select that, it gives three options. They match up with the type files you can download and import into the game. Say you've downloaded a g3x file. When you use the game's import function and select the g3x import function, it'll ask you to browse to wherever it is that you downloaded that file. You point to it and basically it's done.