There's something odd with those computers, their fonts or their install of filemaker pro. Typing in an @ symbol should be the same for all operating systems.
Tests you can try:
Try opening wordpad, notepad or MS Word. Can you type in an @ symbol? (Rules out most issues with the operating system.)
Open filemaker and create a new databse with at least one text field. Try entering a @ symbol in it. If if works, something is wrong with your original database file. It may be corrupted.