Double click to fill down doesn't work

Double click fill down does work, but there must be data in the adjacent, left-most column, so Excel knows how far to copy data down.

I'm not even sure how you ever got that to work. I've never heard of that before.

Of course, the Fill Down command is available as a menu item, but I've always found it most convenient to assign the Command+D keystroke command to it. It actually was one of the default keystroke commands in older versions of Excel, but I think they changed it to Control+D in the first version of Excel for OS X.

Anyhow, you can set the Fill Down command to work with any keystroke combination you want in the Customize Toolbars and Menus dialog:

• Under the View menu, select Toolbars, and then choose Customize Toolbars and Menus... from the submenu to open the Customize Toolbars and Menus dialog.
• Click on the Keyboard... button at the bottom right edge of the dialog to open the Customize Keyboard dialog.
• Select the Edit category on the left, and then scroll down the list of Edit commands on the right and select the Down command.
• Place the cursor in the Press new shortcut key text box, and type whatever keystroke combination you want to use.
• If the keystroke combination is already in use by another command, Excel will tell you that it's currently assigned and will tell you what function it's assigned to. If you want to override the current assignment, click the Add button. Excel will display a prompt asking you whether you want to reassign the key combination to the selected command. If so, click OK, or click Cancel to return to the dialog and select another key combination.
• When you're done, click OK on both dialogs to close them.

Sorry that I can't tell you how to make the double-click work. I guess there might be a way to write a macro using mouse actions for something like that, but as far as I know, there's no way to assign mouse actions to Edit functions in the normal user-definable interface features in Excel.

On my MacBook Pro fill down Click on a cell move the mouse over to right lower corner.

the cursor will change to a cross-hair (or plus sign) click mouse button or click trackpad down and Drag down, or across until desired position and let go when finished  see this movie:

http://screencast.com/t/IH7K5wNY