Which language do you use in your existing sites? If you changed to something other than english (eng-GB), this will cause a problem with the installer, because the installer currently only has support for "english".
If this is your case, you can
- keep trying ;) , because at some moment, the installer will have "translated" (copied) all strings from english to your language - check site.ini.append.php for something like "Locale" (not "ContentLocale") and change this to "eng-GB", before you run the installer. You can change it back to you language later.