Before Minecraft 1.6.1, Mac users could right click with Control + click to use an Item on Minecraft.
On a Mac, control+click is an alternate right click. Since 1.6.1 you can't.
If Control + Click is an alternate way to right click then when you configure a right click to be the use item button you should be able to use control + click too.
37th Duplicate of MC-13695, please use the search function to see if your bug has already been submitted. Currently over 58% of tickets are being closed as duplicate.