Do you´ve din font install?
Take a look and read what he just posted... yes, the DIN font add-on IS installed.
@Cocò: Sometimes there is a problem with the text rendering in OMSI, especially with small font sizes. Someone once posted a possible solution, but I'm not sure if this has been tested - I think it had something to do with uneven font sizes.
But actually it's not a problem with the add-on itself, it has something to do with OMSI, as it works for approximately 95% of all users. Could also be that your graphics card driver is causing trouble there - sorry, but I'm afraid I can't help you with that