Where is the option to mask? I am using the trial. My workflow is to export to tiff 16 bit after camera raw adjustments. Then open tiff in ai sharpen, and do that work, and save. then open tiff in lightroom (for DAM) and final local adjustments and print.
Sharpen AI itself does not yet have a masking capability, or another means to select regions of interest. It was requested by several beta testers, included myself, but it didn't make it yet in this first version. I hope they will add such capability, especially for the Focus modus and the Stabilize modus.
So, for the time being, you'll have to use an image editor that allows masking, and apply Sharpen AI to a duplicate layer. The plugin installs in e.g. Affinity Photo, and in Photoshop CS6 and later, under the plugin filters.
Cheers,
Bart