Unfortunately due to the way Web Platform Installer works it requires SQL Authentication Mode to be used when performing software installations. The message you receive from the Web Platform Installer means that the actual 'sa' account password and the password you entered in the 'Database Administrator Password' textbox (in the Web Platform Installer Wizard) do not match.
You are also not required to create an empty database for MonoX installation via Web Platform Installer the program does that for you.
Please refer this
article how to install MonoX using Web Platform Installer. You will be interested in the 2nd part of the article.