We've just pushed up Acorn 4.5.5 for direct customers, and the App Store version will be on its way in a couple of weeks (hopefully sooner though - maybe it'll fly through the Apple app review process?).
This release is mostly bug fixes, but it also includes a sharing extension which will work on 10.10 Yosemite. This extension works great with the Photos app, which doesn't have the ability to work with other applications via an "Edit In…" (which its older siblings iPhoto and Aperture had). The sharing solution isn't perfect, but it's the best solution for the time being.
You might have also noticed things have been very quiet around here lately. We've been heads down, working hard on some great new stuff for Acorn. And we hope to show it off sometime in the near future.