Have you set up your Kobo Mini, and put in your Kobo registration details? You won't be able to do anything before you do that.
There's a bit of contention over whether the Mini reads .mobi adequately. Either way, it's a simple matter to highlight all your books in Calibre, and convert to epub, which is a known quantity and which the Kobo will handle well. (While you're converting, you might want to strip any hardocoded margin, line spacing, and font size settings - in the Conversion settings, they're under Look and Feel > Filter Style Information.)
You'll need to disinfect any DRM-infested files before you do this.