Looks like you've found where to set the mask in the Spatial Analyst extension.
On your next task, if your not deadset on getting the area from the actual raster cells themselves, I'd use the Spatial analyst 'Raster to Feature' function to get a shapefile or feature class of the polys that you need the acreage of. From the Spatial analyst menu, you want to choose 'convert'
Once you have a poly shapefile, you can download xtools 3.0 from the Arcscripts page if you don't already have it and use it to calculate acreage.
If you make a gdb featureclass, it will have a field called shape_area and this will have the areas of all the polys, you'd just need to do a little unit conversion from there.
If you need to keep it in a raster to get your acreages, take a look at this thread, maybe it'll help.