To edit the layer instead of the layer mask, select it by clicking its thumbnail in the Layers panel. A border appears around the layer thumbnail.
To paste a copied selection into a layer mask, Alt-click (Windows) or Option-click (Mac OS) the layer mask thumbnail in the Layers panel to select and display the mask channel. Choose Edit > Paste, then Select > Deselect. The selection is converted to grayscale and added to the mask. Click the layer thumbnail in the Layers panel to deselect the mask channel.