You can’t subscribe to ANY Kindle Periodicals on Kindle for Mac (that’s not a Lightspeed-specific issue), but you can buy the individual issues of Lightspeed as regular Kindle ebooks. For instance, here’s the February issue:

http://www.amazon.com/Lightspeed-Magazine-February-2013-ebook/dp/B00B93BGFK/

Why that is, I don’t know. I expect Amazon will eventually update the app so that Kindle for Mac users can also use Periodicals. FWIW, the Periodicals program on Amazon is still in Beta, and there’s a lot about it that’s not quite ideal for readers or publishers.

You can also subscribe to Lightspeed via Weightless Books, but unfortunately, the Personal Docs service ALSO doesn’t work for Kindle for Mac, and that’s the only way to set up your subscription so that it automatically is delivered to your Kindle app every month; so there’s no easy to to subscribe to the magazine using that particular app, but you can buy the individual issues, as I said.

Your best bet with that app is probably to subscribe via Weightless Books (http://www.lightspeedmagazine.com/subscribe/), and sign up to have the Mobi version emailed to you every month. When you get your Mobi file on your Mac, you can just open it with your Kindle for Mac app, and it’ll add it to your library. (You can also just set Kindle for Mac as the default app to open Mobi files, so you’d be able to just double click the Mobi file after downloading it.)

If you have any other questions, please let me know. You can email me privately at john [at] lightspeedmagazine.com if that’s easier.