this could be caused by a couple of things.
You may be trying to use a plane that is not in your FS or
one or more of the planes in your fleet is not referred to correctly. If it lists the planes that are in error  go to there aircraft cfg files and check it to make sure that the line that says (atc_type= type of plane (B727) ) . and you have a line that says (atc_airline=name of air line). I might not have the wording correct on the atc_airline But just use any of the default planes aircraft cfg files and you will see what I am talking about. I have found that alot of planes you download are missing this section and atc will either not know what you are or say you are an experemential. Just make sure your aircraft cfg files look like one of the default aircraft cfg file. This will mor or likely clear it up.

Dive Bomber What you need to do is pick one of the default aircraft (B737-400) for instance click on that folder than double click on the aircraft .cfg: file. Not the air file mimimize that window than open your planes aircraft cfg file drag it to one side or the other then reopen the B737-400 and put it on the other side that way they are side by side look fro the section that have the following lines
atc_airlines=
atc_id=
ui_type=
it is in this section where you set the airline and type of plane you are flying the atc section is what ATC reads.
When you compare the B737-400 to your plane you will more than likely find that there are lines either missing or blank. If neccassary print out booth aircraft cfg files.
It should be obivous what is missing. If you still have problems. Send me a private email and I will guide you through it. I am at work now so I do not have access to FS.   
    Gary M Buska 8)