ok its not too tricky but might take a little practice till you get it spot on.
First up use the grid overlay set to 10px x 10px to give you a working grid pattern.
(enable snap to grid too)
Start with the white rectangle on a new layer. Use the VECTOR shape rather than a raster marquee and drag out to the size you want.
Duplicate that layer (CTRL+J) so you now have 2 white rectangles.
Hide the top rectangle layer as we dont need that just yet.
Change the colour of the remaining rectangle to black and select the pen tool and 'Add Anchor Point Tool'
In the middle of base line of the rectangle, add a point (click where you want the point to be - try and get as close to the middle as you can)
Select the 'Direct Selection Tool' from the menu or press A so that the white cursor arrow is selected.
click on the point you just added and drag it down by a couple of grid lines. This should give you an arc shape at the bottom of your rectangle.
Right click on the layer in the layer palette and select 'Rasterise Layer'
Go to Filter > Blur > Gaussian Blur and set it to 2.8 0r thereabouts.
Turn the top layer back on to visible and you should have a drop shadow effect.
Use the marquee tool to make a full selection of the parts you dont need and delete them.
Drop the base layer opacity down to 20% and nudge the layer up a couple of pixels to get slightly sharper edges.
See if that helps.