Hi Don,
Make sure you're putting your aircraft files in the right place. When you download a new aircraft, it probably comes in a zipped (compressed) folder. Unzip the folder (sometimes you can just double click on it, or try right clicking and looking for an "uncompress" option) and place the entire folder into X-Plane's aircraft folder. You can also put it in one of the sub-folders within "Aircraft" such as "general aviation" or "heavy metal."
Even if you add an aircraft folder while X-Plane is running, you can click on the Re-Scan button in the Open Aircraft screen and X-Plane should find it (as long as it's in the aircraft folder).