Hi,
currently, the only way is to install the SDX Libraries from a DVD drive.
However, there are a few options on how to achieve this.
1) Buy (from about $35-$50) or borrow an external DVD and connect it to your Mac mini.
2) If you have an older Mac nearby that has an optical drive, it can be shared over a network.
3) If you can use any other computer, PC or Mac, with an optical drive and you have an external hard drive, you can choose to install only the libraries to this external hard drive. No need to install the Plugin(s) to that computer and also no need to Authorize it. Once you have the SDX Libraries on your external hard drive, you can then either just run them from this drive or copy the Libraries intact to your internal drive on your Mac mini.
Hope this helps.