The config.ini file that you are referencing would have to be in the same directory as the directory from which you launched the java application. if it's not there, you will have to specify the path to where your config.ini is.
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Is this a file you authored or was created on the computer all ready? If you authored it then you can put it wherever you'd like depending on your needs. For example, if this was a network server and administrators needed a way to change this file remotely, you might place it on a shared drive and tell the server to search there.
Its up to you to decide where to place it to best fit the needs of your application. On the other hand if this is not a file you wrote, we'll need more specifics on what program created it.