Now I know I was really dumb :S
The needed file has to be in the same folder as the project p.e.:
current directory: C:\xy\xz\xx\.
absolute path of file: C:\xy\xz\xx\file.txt
relative path of file: .\file.txt
or
absolute path of file: C:\xy\xz\file.txt
relative path of file: .\..\file.txt
and so on.
Greetings
Painjofshem