I know I shouldn't answer in this way but my thread post I edited yesterday got edited by an admin again, so I'll keep it this way.

I found the solution. I read a lot of times and always put my `<script src=...>`-code line above the `PlaceHolderAdditionalPageHead` line in my page layout. All the answers you provided me actually work. There is just a simple thing what doesn't work (for me). I have to put the source-code line **INTO** the `PlaceHolderAdditionalPageHead`-section, not above it. Just tried it out and my test script said: `jQuery library is loaded` :)

This has made my day! Thanks for your suggestions. Have a nice one!

Stefan