Holding the right mouse button should open the build menu.
When I try this, nothing happens. If it helps, I downloaded it on a mac book pro
Can't say I know much about macs :/ Maybe you can reconfigure it; on the Input tab in the Raft Configuration window you should be able to change RightClick to a different key(double-click on the entry in either the Primary or Secondary column). I have a qwerty keyboard so I changed mine to the 'Q' key.
You hold down with 2 fingers and then you move them to select an item