where should I place a .gitignore file so that all my projects use those settings?
I tried various folders and I can only get it to work if I put it in my project folder, but then the settings (of course) only are applied to that project… not my other projects.
You need to use the command line. The reason is, that you need to place the gitignore file in the parent directory of your Eclipse projects, but you cannot see that directory in the Eclipse workbench (as Eclipse projects are top level entities there).
E.g. you have 2 Eclipse projects, which are stored like this in your repository
\mygitrepo\project1
\mygitrepo\project2
then just add the .gitignore file in the mygitrepo directory.