I have a folder with .adf raster data that I'm trying to import into ArcMap 10.6, but I can't figure out how. I'm trying to do menu File → Add Data → Add Data, but the files are not displayed. Nor do they show up under "Contents" in ArcCatalog, but in preview the raster is displayed.

Online instructions talk about "import", but I don't see any menu item that's called this.

How can I import them?

Data is from this website: 50 m Arc GIS Grid http://www.soest.hawaii.edu/hmrg/multibeam/bathymetry.php