Found it!
I used the wrong Subclipse Update-site and therefore had the wrong Subclipse-Version installed.
Instead of:
http://subclipse.tigris.org/update
which will just install v1.0 not the newest version. Newest is currently 1.8, its obtainable over
http://subclipse.tigris.org/update_1.8.x
File-Entries will work now without any trouble... thanks to everybody who put effort into my problem!