First of all Tell me that you did not unzip addon aircraft straight into FS. If you did this can lead to all kinds of headaches. ALWAYS down load into a seperate folder call it what you want I use HANGER for my folder. By unzipping straight into fs it can create multiple Gauge folders and multiple aircraft folders depending on how it was zipped up. I would suggest you take a look at your fs folder to see if you have any duplicate folders if you do I would suggest that you move the contents into the other folder.Than delete the other one. You have to be careful with the gauge folder as some downloads use a seperate folder within the gauge folder and they must stay there. My advice is to stay away from aircraft download managers it is not that hard to install them your self and believe me it saves a lot of hair if you know what I mean

Someone has a good tutorial here at this site for how to install aircraft. It realy is not that hard
As to why some aircraft do not show up that could be several reasons.
One is the duplicate aircraft folder problem I am not sure what fs would do in that case it could cause it to crash.
Another is the plane is meant for AI and these are not flyable and therefore will not show up.
Another problem is in the aircraft cfg file it uses the wrong name for the .AIR file they have to match exactly.
You really have to be carfull when downloading into fs a bad aircraft folder can cause FS to crash without any messages as to why. All it takes is for a panel to not have a needed gauge and if you have two gauge folders I do not think fs will see them both.
But if you did download straight into fs and now have multiple folders do not fell bad. We all have to learn sometimes.
If you need any further help do not be afraid to ask.
