If I download a lens profile from the adobe lens profile downloader?
Is this new profile automatically integrated into my Lightroom 4.3?
If yes, why can I not find it in the list?
Note: I tried to use it on "jpeg" pics from a friend.
Lens profiles can be for raw or not-raw. This is indicated in the lens-profile-downloader for each profile.
A jpg may have been corrected already in the camera so using a raw profile for a jpg may double-correct some things.
If you want to hack things a little, you can use a text-editor to make a copy of the lens-profile .lcp file and change the lines that say < stCamera:CameraRawProfile > true to false instead of true and it'll work for JPGs, then.
Not sure about a Mac, but on Windows these downloaded profiles are under c:\users\username\appdata\roaming\adobe\cameraraw\lensprofiles