Bug in Minecraft with placing and deleting blocks/items

The day when i first got my Mac I downloded Minecraft and when i tried to place a blcok it would not let me it would only let me destroy the block even if i right clicked or left clicked.

What I extected to happen was:
I was going to hit right click to place down the block/use the block, and hit left click to destory the block/item.

What actually happen was:
I hit right click to destory my tree then i crafted it into wodden planks and then i tried to place down the planks to bulid my house by left click