When you load a plane and find yourself with either the Cessna panel or sounds (or both) it means that either panel or sounds (or both) folders have no .cfg file inside, and if said .cfg file is there, then it means that there's some fundamental error in it or because of your meddling.

Usually the problem lies in the fact that those planes have both their panel and sounds aliased to default planes (instead of being shipped with adequate ones, both because of weight of the download and because you can always put a custom panel or soundset in it yourself too), but if you changed the name of the folder of the default 737 (used for the sounds, and in the case of twin engined planes like the MD80 for the panel too) or changed the name of the folder of the default 747 (used for the panel in 4 engined add-on planes like the 707) you find yourself using the default Cessna sounds or panel (or both).
Look into the add-on plane's sound folder. There you'll find a "sound.cfg" file, open said file and read it. you'll find something like this:
[fltsim]
alias=b737_400\sound
The structure of the part after the alias command MUST point to an existing folder.
The same is true into a panel.cfg. You'll find something like:
[fltsim]
alias=b737_400\panel
And here too the alias must point to an existing folder.
If the expected folder(s) is(are) not there or has(have) another name, then it's Cessna time.

There's but one real cure for human stupidity. It's called DEATH.
At the moment mourning the assassination of sarcasm and irony for the good of the "higher".
Proud FSIX user. Active user of FS98, X-plane and novic